Fix PHP 8.1 "strlen(null)" exception creating a Paste without content in Conduit paste.create

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.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=df6c315ace5f), phorge(head=thisThis, ref.master=7cffe557ac24, ref.thisThis=529790613a86)
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/paste/conduit/PasteCreateConduitAPIMethod.php:46]
```

Closes T15613

Test Plan: Create a paste in the deprecated API paste.create without content via Conduit. Shows `error_code ERR-NO-PASTE` as expected, and no exception.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15613

Differential Revision: https://we.phorge.it/D25405