Fix PHP 8.1 "strlen(null)" exceptions to render the Account Creation page

Summary:
Fix numerous PHP 8.1 "strlen(null)" exceptions which block rendering the initial Account
Creation page in a fresh Phorge installation.

The strlen() was used in Phabricator to check if a generic value was a non-empty string.
For this reason, Phorge adopts phutil_nonempty_string() that checks that.

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

Closes T15279

Test Plan: After these code changes the account creation page got displayed (though without CSS and JS).

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15279

Differential Revision: https://we.phorge.it/D25137