Fix PHP 8.1 issue in DifferentialChangeset getOldStatePathVector()

Summary:
The DifferentialChangeset getOldStatePathVector() method assumes oldFile and filename are set.
This worked under PHP <= 8.0, but fails for PHP >= 8.1 with error messsage

```
strlen(): Passing null to parameter #1 ($string) of type string is deprecated
```

Fixes T15517

Test Plan:
Create a diff in which a new file is added.
This file will have oldFile NULL and filename a string.
View the diff https://my.phorge.site/D1234

Reviewers: O1 Blessed Committers, Matthew

Reviewed By: O1 Blessed Committers, Matthew

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

Maniphest Tasks: T15517

Differential Revision: https://we.phorge.it/D25323

src/__phutil_library_map__.php

@@ -488,6 +488,7 @@ phutil_register_library_map(array(
     'DifferentialChangesetRenderer' => 'applications/differential/render/DifferentialChangesetRenderer.php',
     'DifferentialChangesetSearchConduitAPIMethod' => 'applications/differential/conduit/DifferentialChangesetSearchConduitAPIMethod.php',
     'DifferentialChangesetSearchEngine' => 'applications/differential/query/DifferentialChangesetSearchEngine.php',
+    'DifferentialChangesetTestCase' => 'applications/differential/storage/__tests__/DifferentialChangesetTestCase.php',
     'DifferentialChangesetTestRenderer' => 'applications/differential/render/DifferentialChangesetTestRenderer.php',
     'DifferentialChangesetTwoUpRenderer' => 'applications/differential/render/DifferentialChangesetTwoUpRenderer.php',
     'DifferentialChangesetTwoUpTestRenderer' => 'applications/differential/render/DifferentialChangesetTwoUpTestRenderer.php',
@@ -6501,6 +6502,7 @@ phutil_register_library_map(array(
     'DifferentialChangesetRenderer' => 'Phobject',
     'DifferentialChangesetSearchConduitAPIMethod' => 'PhabricatorSearchEngineAPIMethod',
     'DifferentialChangesetSearchEngine' => 'PhabricatorApplicationSearchEngine',
+    'DifferentialChangesetTestCase' => 'PhabricatorTestCase',
     'DifferentialChangesetTestRenderer' => 'DifferentialChangesetRenderer',
     'DifferentialChangesetTwoUpRenderer' => 'DifferentialChangesetHTMLRenderer',
     'DifferentialChangesetTwoUpTestRenderer' => 'DifferentialChangesetTestRenderer',

src/applications/differential/storage/DifferentialChangeset.php

@@ -325,14 +325,16 @@ final class DifferentialChangeset
 
   public function getOldStatePathVector() {
     $path = $this->getOldFile();
-    if (!strlen($path)) {
+    if (!phutil_nonempty_string($path)) {
       $path = $this->getFilename();
     }
 
-    $path = trim($path, '/');
-    $path = explode('/', $path);
+    if (!phutil_nonempty_string($path)) {
+      return null;
+    }
 
-    return $path;
+    $path = trim($path, '/');
+    return explode('/', $path);
   }
 
   public function getNewStatePathVector() {

src/applications/differential/storage/__tests__/DifferentialChangesetTestCase.php

@@ -0,0 +1,16 @@
+<?php
+
+final class DifferentialChangesetTestCase extends PhabricatorTestCase {
+
+  public function testPhp81() {
+    $diff_change_set = new DifferentialChangeset();
+    try {
+      $old_state_vector = $diff_change_set->getOldStatePathVector();
+      $this->assertTrue(true, 'getOldStatePathVector did not throw an error');
+    } catch (Throwable $ex) {
+      $this->assertTrue(false,
+        'getOldStatePathVector threw an exception:'.$ex->getMessage());
+    }
+  }
+
+}