Fix NullReferenceException. (dotnet#5254)

* Added null check for page in ManagedReferenceDocumentProcessor.cs and corresponding UT.

* Removed unneccessary using statement.

* Renamed variables.

src/Microsoft.DocAsCode.Build.ManagedReference/ManagedReferenceDocumentProcessor.cs

@@ -89,7 +89,7 @@ protected override FileModel LoadArticle(FileAndType file, ImmutableDictionary<s
             }
 
             var page = YamlUtility.Deserialize<PageViewModel>(file.File);
-            if (page.Items == null || page.Items.Count == 0)
+            if (page?.Items == null || page.Items.Count == 0)
             {
                 return null;
             }

test/Microsoft.DocAsCode.Build.ManagedReference.Tests/ManagedReferenceDocumentProcessorTest.cs

@@ -358,6 +358,38 @@ public void SystemKeysListShouldBeComplete()
             }
         }
 
+        [Fact]
+        public void LoadArticleWithEmptyFileShouldWarnAndReturnNull()
+        {
+            var fileWithNoContent = "TestData/mref/FileWithNoContent.yml";
+            var file = new FileAndType(Directory.GetCurrentDirectory(), fileWithNoContent, DocumentType.Article);
+            var processor = new ManagedReferenceDocumentProcessor();
+
+            var listener = TestLoggerListener.CreateLoggerListenerWithPhaseStartFilter(nameof(LoadArticleWithEmptyFileShouldWarnAndReturnNull), LogLevel.Info);
+            try
+            {
+                Logger.RegisterListener(listener);
+
+                FileModel actualFileModel;
+                using (new LoggerPhaseScope(nameof(LoadArticleWithEmptyFileShouldWarnAndReturnNull)))
+                {
+                    actualFileModel = processor.Load(file, null);
+                }
+
+                var warnings = listener.GetItemsByLogLevel(LogLevel.Warning);
+                Assert.Single(warnings);
+                var warning = warnings.Single();
+                Assert.Equal("Please add `YamlMime` as the first line of file, e.g.: `### YamlMime:ManagedReference`, otherwise the file will be not treated as ManagedReference source file in near future.", warning.Message);
+                Assert.Equal(fileWithNoContent, warning.File);
+
+                Assert.Null(actualFileModel);
+            }
+            finally
+            {
+                Logger.UnregisterListener(listener);
+            }
+        }
+
         private void BuildDocument(FileCollection files)
         {
             var parameters = new DocumentBuildParameters