Fix PHP 8.1 "strlen(null)" exceptions trying to authenticate via custom OAuth provider

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.

This change also replaces two other `strlen()` calls, only based on //reading// the code but not based on triggering exceptions.

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(), ava(), phorge(), wmf-ext-misc()
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/applications/auth/adapter/PhutilOAuth1AuthAdapter.php:121]
```

```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(), ava(), phorge(), wmf-ext-misc()
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/applications/auth/adapter/PhutilOAuth1AuthAdapter.php:125]
```

Closes T15588

Test Plan: Go to `/auth/login/mediawiki:whatever/` and at least see no `strlen()` errors anymore (while further issues remain). See T15588 for details.

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

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

Maniphest Tasks: T15588

Differential Revision: https://we.phorge.it/D25373