Fix DifferentialDiff getFieldValuesForConduit PHP 8.1 strlen(null) errors

Summary:
Fix DifferentialDiff getFieldValuesForConduit PHP 8.1 strlen(null) errors by replacing the strlen() calls with phutil_nonempty_string().

Also needed to update DifferentialDiffTestCase.php to being a PhabricatorTestCase, as it was throwing an exception prior to any code changes -
```
EXCEPTION (Exception): Trying to read configuration "policy.locked" before configuration has been initialized.
```

Fixes T15529

Test Plan: arc diff

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

Reviewed By: O1 Blessed Committers, valerio.bozzolan, avivey

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

Maniphest Tasks: T15529

Differential Revision: https://we.phorge.it/D25333

src/__phutil_library_map__.php

@@ -6565,7 +6565,7 @@ phutil_register_library_map(array(
     'DifferentialDiffRepositoryProjectsHeraldField' => 'DifferentialDiffHeraldField',
     'DifferentialDiffSearchConduitAPIMethod' => 'PhabricatorSearchEngineAPIMethod',
     'DifferentialDiffSearchEngine' => 'PhabricatorApplicationSearchEngine',
-    'DifferentialDiffTestCase' => 'PhutilTestCase',
+    'DifferentialDiffTestCase' => 'PhabricatorTestCase',
     'DifferentialDiffTransaction' => 'PhabricatorApplicationTransaction',
     'DifferentialDiffTransactionQuery' => 'PhabricatorApplicationTransactionQuery',
     'DifferentialDiffViewController' => 'DifferentialController',

src/applications/differential/storage/DifferentialDiff.php

@@ -780,7 +780,7 @@ final class DifferentialDiff
     $refs = array();
 
     $branch = $this->getBranch();
-    if (strlen($branch)) {
+    if (phutil_nonempty_string($branch)) {
       $refs[] = array(
         'type' => 'branch',
         'name' => $branch,
@@ -788,7 +788,7 @@ final class DifferentialDiff
     }
 
     $onto = $this->loadTargetBranch();
-    if (strlen($onto)) {
+    if (phutil_nonempty_string($onto)) {
       $refs[] = array(
         'type' => 'onto',
         'name' => $onto,
@@ -796,7 +796,7 @@ final class DifferentialDiff
     }
 
     $base = $this->getSourceControlBaseRevision();
-    if (strlen($base)) {
+    if ($base !== null && strlen($base)) {
       $refs[] = array(
         'type' => 'base',
         'identifier' => $base,
@@ -804,7 +804,7 @@ final class DifferentialDiff
     }
 
     $bookmark = $this->getBookmark();
-    if (strlen($bookmark)) {
+    if (phutil_nonempty_string($bookmark)) {
       $refs[] = array(
         'type' => 'bookmark',
         'name' => $bookmark,

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

@@ -1,6 +1,12 @@
 <?php
 
-final class DifferentialDiffTestCase extends PhutilTestCase {
+final class DifferentialDiffTestCase extends PhabricatorTestCase {
+
+  protected function getPhabricatorTestCaseConfiguration() {
+    return array(
+      self::PHABRICATOR_TESTCONFIG_BUILD_STORAGE_FIXTURES => true,
+    );
+  }
 
   public function testDetectCopiedCode() {
     $copies = $this->detectCopiesIn('lint_engine.diff');
@@ -73,5 +79,35 @@ EODIFF;
     $this->assertTrue(true);
   }
 
+  public function testGetFieldValuesForConduit() {
+
+    $parser = new ArcanistDiffParser();
+    $raw_diff = <<<EODIFF
+diff --git a/src b/src
+index 123457..bb216b1 100644
+--- a/src
++++ b/src
+@@ -1,5 +1,5 @@
+ Line a
+-Line b
++Line 2
+ Line c
+ Line d
+ Line e
+EODIFF;
+
+    $diff = DifferentialDiff::newFromRawChanges(
+      PhabricatorUser::getOmnipotentUser(),
+      $parser->parseDiff($raw_diff));
+    $this->assertTrue(true);
+
+    $field_values = $diff->getFieldValuesForConduit();
+    $this->assertTrue(is_array($field_values));
+    foreach (['revisionPHID', 'authorPHID', 'repositoryPHID', 'refs']
+      as $key) {
+      $this->assertTrue(array_key_exists($key, $field_values));
+    }
+
+  }
 
 }