devserver: return empty host log if not events were logged

This fixes a bug where the /api/hostlog?ip=foo URL returns no value when
no events were logged for a given IP address, which causes a problem on
the client side (who is expecting a valid JSON object). Instead, we're
not returning an empty log.

BUG=None

TEST=Devserver returns empty log (list) for queries about idle or
non-existent hosts

Change-Id: If2b27b02272dcf38b732918115e7bd7bb2e49a7a
Reviewed-on: https://gerrit.chromium.org/gerrit/34825
Reviewed-by: Chris Sosa <[email protected]>
Commit-Ready: Gilad Arnold <[email protected]>
Tested-by: Gilad Arnold <[email protected]>
1 file changed