Fix DifferentialCommitMessageField renderFieldValue PHP 8.1 strlen(null) error

Summary:
arc diff throws strlen(null) error from DifferentialCommitMessageField renderFieldValue when calling a Phorge server running PHP 8.1

Add unit test, which required a new DifferentialTestCommitMessageField class so as to be able to test the abstract DifferentialCommitMessageField class methods.

Fixes T15530

Test Plan:
Make a change in a git repo with remote a Phorge server running PHP 8.1
Run:

```
arc diff
```
See exception thrown as per T15530

Reviewers: O1 Blessed Committers, valerio.bozzolan, avivey

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

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

Maniphest Tasks: T15530

Differential Revision: https://we.phorge.it/D25334

src/__phutil_library_map__.php

@@ -714,6 +714,7 @@ phutil_register_library_map(array(
     'DifferentialTabReplacementTestCase' => 'applications/differential/parser/__tests__/DifferentialTabReplacementTestCase.php',
     'DifferentialTagsCommitMessageField' => 'applications/differential/field/DifferentialTagsCommitMessageField.php',
     'DifferentialTasksCommitMessageField' => 'applications/differential/field/DifferentialTasksCommitMessageField.php',
+    'DifferentialTestCommitMessageField' => 'applications/differential/field/DifferentialTestCommitMessageField.php',
     'DifferentialTestPlanCommitMessageField' => 'applications/differential/field/DifferentialTestPlanCommitMessageField.php',
     'DifferentialTestPlanField' => 'applications/differential/customfield/DifferentialTestPlanField.php',
     'DifferentialTitleCommitMessageField' => 'applications/differential/field/DifferentialTitleCommitMessageField.php',
@@ -6760,6 +6761,7 @@ phutil_register_library_map(array(
     'DifferentialTabReplacementTestCase' => 'PhabricatorTestCase',
     'DifferentialTagsCommitMessageField' => 'DifferentialCommitMessageField',
     'DifferentialTasksCommitMessageField' => 'DifferentialCommitMessageField',
+    'DifferentialTestCommitMessageField' => 'DifferentialCommitMessageField',
     'DifferentialTestPlanCommitMessageField' => 'DifferentialCommitMessageField',
     'DifferentialTestPlanField' => 'DifferentialCoreCustomField',
     'DifferentialTitleCommitMessageField' => 'DifferentialCommitMessageField',

src/applications/differential/field/DifferentialCommitMessageField.php

@@ -60,7 +60,7 @@ abstract class DifferentialCommitMessageField
   }
 
   public function renderFieldValue($value) {
-    if (!strlen($value)) {
+    if (!phutil_nonempty_string($value)) {
       return null;
     }
 

src/applications/differential/field/DifferentialRevisionIDCommitMessageField.php

@@ -72,7 +72,7 @@ final class DifferentialRevisionIDCommitMessageField
   }
 
   public function renderFieldValue($value) {
-    if (!strlen($value)) {
+    if (!phutil_nonempty_string($value)) {
       return null;
     }
 

src/applications/differential/field/DifferentialTestCommitMessageField.php

@@ -0,0 +1,10 @@
+<?php
+
+/**
+ * This class should only be used for unit tests
+ */
+final class DifferentialTestCommitMessageField
+  extends DifferentialCommitMessageField {
+  public function getFieldName() { return 'Test'; }
+  public function getFieldOrder() { return 1; }
+}

src/applications/differential/field/__tests__/DifferentialCommitMessageFieldTestCase.php

@@ -28,4 +28,21 @@ final class DifferentialCommitMessageFieldTestCase
     unset($env);
   }
 
+  public function testRenderFieldValue() {
+    $test_object = new DifferentialTestCommitMessageField();
+    $this->assertEqual('foo', $test_object->renderFieldValue('foo'),
+      'Normal strings should be rendered unaltered');
+
+    $this->assertEqual(null, $test_object->renderFieldValue(''),
+      'Empty strings should be returned as null');
+
+    $this->assertEqual(null, $test_object->renderFieldValue(null),
+      'null values strings should be returned as null');
+
+    $test_object = new DifferentialRevisionIDCommitMessageField();
+    $expected = 'http://phabricator.example.com/D123';
+    $this->assertEqual($expected, $test_object->renderFieldValue('123'));
+    $this->assertEqual(null, $test_object->renderFieldValue(null));
+  }
+
 }