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:
283b280320/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.

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,