Fix a log warning when searching for ranges on custom "Date" fields

Summary:
See https://discourse.phabricator-community.org/t/traceback-rendering-task-query-in-dashboard/2450/.

It looks like this blames to D19126, which added some more complex constraint logic but overlooked "range" constraints, which are handled separately.

Test Plan:

• Added a custom "date" field to Maniphest with "search": true.
• Executed a range query against the field.

Then:

• Before: Warnings about undefined indexes in the log.
• After: No such warnings.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: jbrownEP

Differential Revision: https://secure.phabricator.com/D20225