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