Fix PHP 8.1 "strlen(null)" exceptions creating an 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.

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuth1AuthProvider.php:163]
```

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuth1AuthProvider.php:178]
```

```
ERROR 8192: strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/auth/provider/PhabricatorOAuthAuthProvider.php:147]
```

Closes T15912

Test Plan: As an admin, create an OAuth provider (such as using Bitbucket) and check the error logs or Dark Console.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15912

Differential Revision: https://we.phorge.it/D25771

src/applications/auth/provider/PhabricatorOAuth1AuthProvider.php

···
 
     switch ($key) {
       case self::PROPERTY_CONSUMER_KEY:
-        if (strlen($old)) {
+        if (phutil_nonempty_string($old)) {
           return pht(
             '%s updated the OAuth consumer key for this provider from '.
             '"%s" to "%s".',
···
             $new);
         }
       case self::PROPERTY_CONSUMER_SECRET:
-        if (strlen($old)) {
+        if (phutil_nonempty_string($old)) {
           return pht(
             '%s updated the OAuth consumer secret for this provider.',
             $xaction->renderHandleLink($author_phid));

src/applications/auth/provider/PhabricatorOAuthAuthProvider.php

···
 
     switch ($key) {
       case self::PROPERTY_NOTE:
-        if (strlen($old)) {
+        if (phutil_nonempty_string($old)) {
           return pht(
             '%s updated the OAuth application notes for this provider.',
             $xaction->renderHandleLink($author_phid));