R1:538cccc63e0e

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…