R1:f704f905d260

Let PhabricatorSearchCheckboxesField survive saved query data with mismatched types

Summary:
Fixes T12851.

This should fix the error I'm seeing, which is:

* `Argument 1 passed to array_fuse() must be of the type array, boolean given`

There may be a better way to patch this up than overriding the getValue() method,
however.

Test Plan:
- Changed the default "Tags" filter to specify `true` instead of `array('self')`, then viewed that filter in the UI.
- Before patch: fatal.
- After patch: page loads.