Correct override_urls check in Api (closes #692)

This applies the same change which #516 made to Resource to only issue
the deprecation warning when override_urls() actually returns something.

This also changes the logic in both locations to avoid calling
override_urls() more than once
Merge pull request #819 from acdha/692-correct-override_urls-warning

Correct override_urls check in Api (closes #692)