Allow free allowance to be set to 0

We want to be able to set the free allowance for a service to 0, but the
form was not allowing this - it gave an error message of `Cannot be
empty`. This can be fixed by changing the WTForms validator from
`DataRequired` (which coerces 0 to falsey) to the `InputRequired`
validator.

app/main/forms.py

@@ -40,7 +40,14 @@ from wtforms import (
     ValidationError,
     validators,
 )
-from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
+from wtforms.validators import (
+    URL,
+    DataRequired,
+    InputRequired,
+    Length,
+    Optional,
+    Regexp,
+)
 
 from app.formatters import format_thousands, guess_name_from_email_address
 from app.main.validators import (
@@ -1300,7 +1307,7 @@ class FreeSMSAllowance(StripWhitespaceForm):
     free_sms_allowance = GovukIntegerField(
         'Numbers of text message fragments per year',
         validators=[
-            DataRequired(message='Cannot be empty')
+            InputRequired(message='Cannot be empty')
         ]
     )