[sql lab] fix impersonation + template issue (#3644)

When the database impersonation flag is on, a query using a template
fails. It has to do with templating using a database connection without
a username being specified by the caller, along with the fact that the
work is taking place on a worker, outside a web request, where
referencing g.user raises this exception.

superset/models/core.py

@@ -598,7 +598,12 @@ class Database(Model, AuditMixinNullable):
             params['poolclass'] = NullPool
         uri = self.db_engine_spec.adjust_database_uri(uri, schema)
         if self.impersonate_user:
-            uri.username = user_name if user_name else g.user.username
+            eff_username = uri.username
+            if user_name:
+                eff_username = user_name
+            elif hasattr(g, 'user') and g.user.username:
+                eff_username = g.user.username
+            uri.username = eff_username
         return create_engine(uri, **params)
 
     def get_reserved_words(self):