Fix PHP 8.1 "strlen(null)" exception setting Story Points on a Task

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. Here we adopt phutil_string_cast() to
reply to the question "is this an empty string?".

Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_string_cast() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.

Closes T15390

Test Plan:
Enable maniphest.points in settings, for example with:

./bin/config set maniphest.points --stdin <<< '{"enabled":true}'

Then try to create a Task: it does not explode anymore in PHP 8.1+.

Also try to set various possible values from the Conduit API method "maniphest.edit".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15390

Differential Revision: https://we.phorge.it/D25222