Fix ‘help’ appearing when it shouldn’t

Steps to reproduce:
- make a template with a placeholder
- click ‘send yourself a test’
- leave fields blank
- click ‘check’
- see error, click ‘back’

Expected: previous page

Actual: previous page with blue help sidebar

When the URL contains `help=0`, `request.args.get('help')` returns '0'.
Doing `if '0':` is the same as doing any `if <non empty string>:` which
returns `True`.

So we should only display the help when the help query parameter is:

- not missing
- AND a string that isn’t `'0'`

app/main/views/send.py

@@ -243,7 +243,7 @@ def check_messages(service_id, template_type, upload_id):
     )
 
     if request.args.get('from_test') and len(template.placeholders):
-        extra_args = {'help': 1} if request.args.get('help') else {}
+        extra_args = {'help': 1} if request.args.get('help', '0') != '0' else {}
         back_link = url_for(
             '.send_test', service_id=service_id, template_id=template.id, **extra_args
         )