Use unambiguously magic value for None choices

WTForms coerces `None` as a choice to `'None'` as a string when
rendering form fields (form fields will only ever have string data
because that what the browser posts back).

But internally WTForms coerces `None` to mean an unset value, ie where
the user hasn’t selected a radio button:
https://github.com/wtforms/wtforms/blob/283b2803206825158834f1828bbf749c129b7c47/src/wtforms/utils.py#L1-L20

We shouldn’t use `None` to mean two different things. And in fact we
can’t, because it in effect means that we’re always getting a value
for the `move_to` field, even if the user hasn’t chosen to move any
templates. Which results in some very expected behaviour.

app/main/forms.py

@@ -707,24 +707,26 @@ class ServicePostageForm(StripWhitespaceForm):
 
 class FieldWithNoneOption():
 
-    # This needs to match the data the browser will post from
-    # <input value='None'>
-    NONE_OPTION_VALUE = 'None'
+    # This is a special value that is specific to our forms. This is
+    # more expicit than casting `None` to a string `'None'` which can
+    # have unexpected edge cases
+    NONE_OPTION_VALUE = '__NONE__'
 
+    # When receiving Python data, eg when instantiating the form object
+    # we want to convert that data to our special value, so that it gets
+    # recognised as being one of the valid choices
+    def process_data(self, value):
+        self.data = self.NONE_OPTION_VALUE if value is None else value
+
+    # After validation we want to convert it back to a Python `None` for
+    # use elsewhere, eg posting to the API
     def post_validate(self, form, validation_stopped):
-        if self.data == self.NONE_OPTION_VALUE:
+        if self.data == self.NONE_OPTION_VALUE and not validation_stopped:
             self.data = None
 
 
 class RadioFieldWithNoneOption(FieldWithNoneOption, RadioField):
-
-    def validate(self, *args, **kwargs):
-
-        if self.data == self.NONE_OPTION_VALUE:
-            self.data = None
-            return True
-
-        return super().validate(*args, **kwargs)
+    pass
 
 
 class HiddenFieldWithNoneOption(FieldWithNoneOption, HiddenField):
@@ -1194,10 +1196,13 @@ class TemplateAndFoldersSelectionForm(Form):
         self.op = None
         self.is_move_op = self.is_add_folder_op = self.is_add_template_op = False
 
+        if current_folder_id is None:
+            current_folder_id = RadioFieldWithNoneOption.NONE_OPTION_VALUE
+
         self.move_to.choices = [
             (item['id'], item['name'])
             for item in ([self.ALL_TEMPLATES_FOLDER] + all_template_folders)
-            if item['id'] != str(current_folder_id)
+            if item['id'] != current_folder_id
         ]
 
         self.add_template_by_template_type.choices = list(filter(None, [
@@ -1237,5 +1242,6 @@ class TemplateAndFoldersSelectionForm(Form):
     move_to_new_folder_name = StringField('Folder name', validators=[required_for_ops('move_to_new_folder')])
 
     add_template_by_template_type = RadioFieldWithNoneOption('Add new', validators=[
+        Optional(),
         required_for_ops('add_template')
     ])

tests/app/main/views/test_service_settings.py

@@ -2188,12 +2188,12 @@ def test_set_postage_saves(
 
 @pytest.mark.parametrize('current_branding, expected_values, expected_labels', [
     (None, [
-        'None', '1', '2', '3', '4', '5',
+        '__NONE__', '1', '2', '3', '4', '5',
     ], [
         'GOV.UK', 'org 1', 'org 2', 'org 3', 'org 4', 'org 5'
     ]),
     ('5', [
-        '5', 'None', '1', '2', '3', '4',
+        '5', '__NONE__', '1', '2', '3', '4',
     ], [
         'org 5', 'GOV.UK', 'org 1', 'org 2', 'org 3', 'org 4',
     ]),
@@ -2314,7 +2314,8 @@ def test_should_preview_email_branding(
 
 @pytest.mark.parametrize('posted_value, submitted_value', (
     ('1', '1'),
-    ('None', None),
+    ('__NONE__', None),
+    pytest.param('None', None, marks=pytest.mark.xfail(raises=AssertionError)),
 ))
 def test_should_set_branding_and_organisations(
     logged_in_platform_admin_client,

tests/app/main/views/test_template_folders.py

@@ -901,7 +901,7 @@ def test_should_be_able_to_move_a_sub_item(
         template_folder_id=PARENT_FOLDER_ID,
         _data={
             'operation': 'move_to_existing_folder',
-            'move_to': 'None',
+            'move_to': '__NONE__',
             'templates_and_folders': [GRANDCHILD_FOLDER_ID],
         },
         _expected_status=302,
@@ -929,6 +929,13 @@ def test_should_be_able_to_move_a_sub_item(
         'move_to_new_folder_name': 'foo',
         'move_to': PARENT_FOLDER_ID
     },
+    # move to existing, but no templates to move
+    {
+        'operation': 'move_to_existing_folder',
+        'templates_and_folders': [],
+        'move_to_new_folder_name': '',
+        'move_to': PARENT_FOLDER_ID
+    },
     # move to new, but nothing selected to move
     {
         'operation': 'move_to_new_folder',
@@ -944,6 +951,14 @@ def test_should_be_able_to_move_a_sub_item(
         'move_to': PARENT_FOLDER_ID,
         'add_template_by_template_type': 'email',
     },
+    # add a new template, but also move to root folder
+    {
+        'operation': 'add_template',
+        'templates_and_folders': [],
+        'move_to_new_folder_name': '',
+        'move_to': '__NONE__',
+        'add_template_by_template_type': 'email',
+    },
 ])
 def test_no_action_if_user_fills_in_ambiguous_fields(
     client_request,