Fix PHP 8.1 "strlen(null)" exception trying to create an empty Menu Item

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_string() as a replacement.

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

Closes T15398

Test Plan:
Applied this change and "Edit Menu Item" overlay dialog correctly shows "You must choose a project" resp. "You must choose a room." error message, now without an exception.

Go to /home/menu/configure/custom/ and also to /home/menu/configure/global/ and try:

• Add Label with name 0 / 1 / 1.123 etc.
• Add Label, Save again
• Add Link, Save again
• Add Application, Save again
• Add Divider, Save again
• Add Conpherence, Save again
• Add Link, Save Again
• Add Motivator (Cat Facts), Save again
• Add Form, Save again
• Order Items
• Activate/Disable Built-in Home (unrelated)
• Remove all the above

Also, tried on a Project menu.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15398

Differential Revision: https://we.phorge.it/D25230