Fix issue strategy returning NULL

When a file strategy returns null in an error situation the factory
should filter these null values. This allows the users of the library
to add middleware that catches any errors during the parsing process
and return null to continue with the process of other files.

src/phpDocumentor/Reflection/Php/ProjectFactory.php

@@ -76,7 +76,9 @@ public function create($name, array $files): ProjectInterface
         foreach ($files as $filePath) {
             $strategy = $this->strategies->findMatching($filePath);
             $file = $strategy->create($filePath, $this->strategies);
-            $project->addFile($file);
+            if ($file !== null) {
+                $project->addFile($file);
+            }
         }
 
         $this->buildNamespaces($project);

tests/unit/phpDocumentor/Reflection/Php/ProjectFactoryTest.php

@@ -187,6 +187,30 @@ public function testSingleFileMultipleNamespaces()
         $this->assertCount(1, $namespaces['\mySpace']->getClasses());
     }
 
+    public function testErrorScenarioWhenFileStrategyReturnsNull()
+    {
+        $fileStrategyMock = m::mock(ProjectFactoryStrategy::class);
+        $fileStrategyMock->shouldReceive('matches')->twice()->andReturn(true);
+        $fileStrategyMock->shouldReceive('create')
+            ->twice()
+            ->andReturnValues(
+                [
+                    null,
+                    new File(md5('some/other.php'), 'some/other.php'),
+                ]
+            );
+
+        $projectFactory = new ProjectFactory([$fileStrategyMock]);
+
+        $files = ['some/file.php', 'some/other.php'];
+        $project = $projectFactory->create('MyProject', $files);
+
+        $this->assertInstanceOf(Project::class, $project);
+
+        $projectFilePaths = array_keys($project->getFiles());
+        $this->assertEquals(['some/other.php'], $projectFilePaths);
+    }
+
     /**
      * Uses the ProjectFactory to create a Project and returns the namespaces created by the factory.
      *