handlers/front: fix sqlalchemy query

If home_node_ids is empty, we can get:
SAWarning: The IN-predicate on "topic.node_id" was invoked with an empty
sequence. This results in a contradiction, which nonetheless can be expensive
to evaluate.  Consider alternative strategies for improved performance.
  return o[0](self, self.expr, op, *(other + o[1:]), **kwargs)

Querying on an empty sequence is expensive.

Signed-off-by: Wei Liu <liuw@liuw.name>
Merge pull request #79 from liuw/hotfix/homepage-query-clean-up