eina/hash: optimize eina_hash_find() when hash is empty.

Summary:
Check if hash is empty before computing the hash key and look inside the
hash to find data.

Note: could have called the eina_hash_population() api but didn't

because of extra function call.

Reviewers: Hermet

Reviewed By: Hermet

Subscribers: cedric, reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D9531