Stop letting people enter empty SMS senders

This shouldn’t be deployed until we’ve migrated the database to default
to GOVUK.

app/main/forms.py

@@ -492,6 +492,7 @@ class ServiceSmsSender(Form):
     sms_sender = StringField(
         'Text message sender',
         validators=[
+            DataRequired(message="Can’t be empty"),
             Length(max=11, message="Enter 11 characters or fewer")
         ]
     )