endpoints/api: Allow null fields in user metadata

The user metadata fields are nullable in the database, but were not in
the json sechema.  This prevented users from updating some of their
information on the site if they hadn't set the metadata fields.

util/saas/useranalytics.py

@@ -97,7 +97,7 @@ class _MarketoAnalyticsClient(object):
       lookupField='id',
     )
 
-  def change_metadata(self, email, given_name, family_name, company, location):
+  def change_metadata(self, email, given_name=None, family_name=None, company=None, location=None):
     lead_data = self._get_lead_metadata(given_name, family_name, company, location)
     if not lead_data:
       return