Raise a tailored error when calling "transaction.search" with empty "phids" constraint

Summary:
Ref T13164. See PHI725. For real "*.search" methods, parameters get validated and you get an error if you use an empty list as a constraint.

Since "transaction.search" isn't really a normal "*.search" method, it doesn't benefit from this. Just do the check manually for now.

Test Plan: Made transaction.search calls with no constraints (got results); a valid costraint (got fewer results); and an invalid empty constraint (got an exception).

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19562