Fix PHP 8.1 "strlen(null)" exception creating a Portal
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.
```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/dashboard/menuitem/PhabricatorDashboardPortalMenuItem.php:34]
```
Closes T15847
Test Plan: Go to `/portal/edit/form/default/`, set a name and click `Create Portal`.
Reviewers: O1 Blessed Committers, valerio.bozzolan
Reviewed By: O1 Blessed Committers, valerio.bozzolan
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T15847
Differential Revision: https://we.phorge.it/D25681