Fix PHP 8.1 "strlen(null)" exception which blocks rendering error pages for Packages' Create Package and Fund's Create Initiative

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 T15365

Test Plan: Applied this change and `/packages/package/edit/form/default/` and `/fund/create/` finally rendered in web browser, showing the expected error messages about not having entered any data in the creation page.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15365

Differential Revision: https://we.phorge.it/D25202