The API for the review request draft was generating a query incorrectly.

Setting an empty target_groups value would result in all available

groups on the global or Local Site to appear, meaning it was impossible

to clear out the list. This problem was specific to target_groups,

though other fields (target_people and depends_on) performed

effectively useless queries as well.
This change fixes this to check whether the list of values is empty

before doing any real work. If it is empty, then empty values are

immediately returned.

Unit tests pass.
Reproduced the issue locally and verified the fix.