R1:cef12d8dc202

Fix PHP 8.1 "strlen(null)" exceptions editing a form when custom field of type Date exists

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1. Phorge adopts `phutil_nonempty_scalar()` as a replacement when both string and integers could be passed as a value like here.

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_scalar() throws an…