Allow Events without traits to be returned

Due to the join with Traits done in the SqlAlchemy implementation
of get_events, only Events with Traits can be returned from that
call. This would be expected behavior if Traits are provided in
the filter, but if no Traits are provided, all matching Events
should be returned. Even those without Traits.

The solution to this is to pre-populate the dictionary of Event
models before populating their Traits in the case of no Trait
filters being provided.

Change-Id: I4b0a5c20952f1dd5f2e2db631ffe1a14bd168fb3
Closes-Bug: #1245992
Merge "Allow Events without traits to be returned"