On-behalf hook_uuid_entities_export_entity_alter()s look for nonexistent properties

In uuid.core.inc, there are on-behalf implementations for user and node entities that make sure the property is truly not set (isset() will return false if the value is null, so we have to check for that case), unsetting it in the export if it isn't. However, notices will be thrown when iterating through node properties when the comment module is disabled:

Notice: Undefined property: stdClass::$last_comment_timestamp in node_uuid_entities_features_export_entity_alter() (line 285 of /path/to/uuid/uuid.core.inc).

We have to make sure the properties aren't null, but we aren't checking for the case in which the property doesn't actually exist, causing the warning when we try to reference it. Pairing the condition with a call to property_exists() fixes the problem. Patch to follow shortly.