Fix a PHP 8.1 deprecated use of strlen with a NULL argument

Summary:
This strlen() call was preventing a new Phorge instance to be deployed/configured.
Indeed, on a fresh instance, configuration's "base-uri" key may not be defined witch lead to a Runtime Exception.
Using strlen() to check string validity 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.

Fix T15605

Test Plan:

• Checkout a fresh Phorge local copy from official 'https://we.phorge.it/source/phorge.git'
• Install/Configure local webserver/database
• Open http://phorge.domain in you browser
• Configure Phorge database (as requested by webpage)
• Create Phorge database (as requested by webpage)
• You should be able to reach administrator account page instead of getting a RuntimeException

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15605

Differential Revision: https://we.phorge.it/D25394