Fix PHP 8.1 "strlen(null)" exception which blocks creating personal and global Herald rules

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 T15348

Test Plan:
Applied this change (on top of D25157) and creation form at `/herald/edit/?content_type=HeraldPreCommitRefAdapter&rule_type=personal` rendered in web browser.
Same applies for other types of personal and global Herald rules.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

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

Maniphest Tasks: T15348

Differential Revision: https://we.phorge.it/D25190

src/infrastructure/contentsource/PhabricatorContentSource.php

@@ -81,6 +81,13 @@ abstract class PhabricatorContentSource extends Phobject {
       ));
   }
 
+  /**
+   * Get the internal source name
+   *
+   * This is usually coming from a SOURCECONST constant.
+   *
+   * @return string|null
+   */
   final public function getSource() {
     return $this->source;
   }

src/infrastructure/contentsource/PhabricatorUnknownContentSource.php

@@ -7,7 +7,7 @@ final class PhabricatorUnknownContentSource
 
   public function getSourceName() {
     $source = $this->getSource();
-    if (strlen($source)) {
+    if ($source) {
       return pht('Unknown ("%s")', $source);
     } else {
       return pht('Unknown');