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.

endpoints/api/test/test_user.py

@@ -0,0 +1,42 @@
+import pytest
+
+from mock import patch
+
+from endpoints.api.test.shared import conduct_api_call
+from endpoints.api.user import User
+from endpoints.test.shared import client_with_identity
+from features import FeatureNameValue
+
+from test.fixtures import *
+
+
+def test_user_metadata_update(client):
+  with patch('features.USER_METADATA', FeatureNameValue('USER_METADATA', True)):
+    with client_with_identity('devtable', client) as cl:
+      metadata = {
+        'given_name': 'Quay',
+        'family_name': 'User',
+        'location': 'NYC',
+        'company': 'Red Hat',
+      }
+
+      # Update all user metadata fields.
+      conduct_api_call(cl, User, 'PUT', None, body=metadata)
+
+      # Test that they were successfully updated.
+      user = conduct_api_call(cl, User, 'GET', None).json
+      for field in metadata:
+        assert user.get(field) == metadata.get(field)
+
+      # Now nullify one of the fields, and remove another.
+      metadata['company'] = None
+      location = metadata.pop('location')
+
+      conduct_api_call(cl, User, 'PUT', None, body=metadata)
+
+      user = conduct_api_call(cl, User, 'GET', None).json
+      for field in metadata:
+        assert user.get(field) == metadata.get(field)
+
+      # The location field should be unchanged.
+      assert user.get('location') == location