Validate name field on ServiceUpdateEmailBranding form

This introduces a validator to validate that the name field is not empty
on the ServiceUpdateEmailBranding form, but only if the form details are
being submitted. If a file is being uploaded, the name is allowed to be
empty.

app/main/forms.py

@@ -848,6 +848,11 @@ class ServiceUpdateEmailBranding(StripWhitespaceForm):
         ]
     )
 
+    def validate_name(form, name):
+        op = request.form.get('operation')
+        if op == 'email-branding-details' and not form.name.data:
+            raise ValidationError('This field is required')
+
 
 class PDFUploadForm(StripWhitespaceForm):
     file = FileField_wtf(