R1:c4f320a7e805

Make standard fields more liberal about interpreting empty strings

Summary:
Fixes T3867. We currently show more empty custom field values on task detail pages than we should, for at least two reasons:

- `<select />` fields with an empty string option store `""`, but users reasonably expect this to mean "no value".
- Old fields may have stored empty strings, and migrated forward.

This fix generally aligns behavior with user expectations. We could get more extreme about not storing `""` in the…