Allow tree photos and collection UDFs in search filters

Fixes #1239
Don't query the ecobenefits service if there are no trees.

It's not possible to get correct SQL from an EmptyQuerySet, so if
there are no trees, we shouldn't try.
Show all zeros for ecobenefits with no trees
Merge pull request #1251 from maurizi/fix/ecobenefits-search

Fix ecobenefits search