Fix some PHP 8.1 "strlen(null)" exceptions on Differential Revision page

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=18554ea76ceb), phorge(head=diff3, ref.master=e11c5486c92b, ref.diff3=e11c5486c92b)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/differential/customfield/DifferentialBranchField.php:42]
```

Closes T15432

Test Plan: After applying these three changes (on top of D25262 and D25263), there is no `strlen()` related exception displayed on `/D1/` anymore, however there are further exceptions to be sorted out in other tasks.

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

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

Maniphest Tasks: T15432

Differential Revision: https://we.phorge.it/D25264

src/applications/differential/customfield/DifferentialBranchField.php

@@ -36,8 +36,8 @@ final class DifferentialBranchField
   }
 
   private function getBranchDescription(DifferentialDiff $diff) {
-    $branch = $diff->getBranch();
-    $bookmark = $diff->getBookmark();
+    $branch = coalesce($diff->getBranch(), '');
+    $bookmark = coalesce($diff->getBookmark(), '');
 
     if (strlen($branch) && strlen($bookmark)) {
       return pht('%s (bookmark) on %s (branch)', $bookmark, $branch);