nmhive.py: Return 400 errors for data-less POSTs

nmhive.py: Return 400 errors for data-less POSTs

Avoid:

  Traceback (most recent call last):
    File "/.../flask/app.py", line 1836, in __call__
      return self.wsgi_app(environ, start_response)
    File "/.../flask/app.py", line 1820, in wsgi_app
      response = self.make_response(self.handle_exception(e))
    File "/.../flask/app.py", line 1403, in handle_exception
      reraise(exc_type, exc_value, tb)
    File "/.../flask/_compat.py", line 33, in reraise
      raise value
    File "/.../flask/app.py", line 1817, in wsgi_app
      response = self.full_dispatch_request()
    File "/.../flask/app.py", line 1477, in full_dispatch_request
      rv = self.handle_user_exception(e)
    File "/.../flask/app.py", line 1381, in handle_user_exception
      reraise(exc_type, exc_value, tb)
    File "/.../flask/_compat.py", line 33, in reraise
      raise value
    File "/.../flask/app.py", line 1475, in full_dispatch_request
      rv = self.dispatch_request()
    File "/.../flask/app.py", line 1461, in dispatch_request
      return self.view_functions[rule.endpoint](**req.view_args)
    File "/.../nmhive.py", line 59, in message_id_tags
      for change in changes:
  TypeError: 'NoneType' object is not iterable