[Syntax] Represent missing optioanl nodes as nullptr (#14300)

Allocating RawSyntax/SyntaxData for missing optional node is a waste of
resource.

tools/SourceKit/lib/SwiftLang/SwiftEditor.cpp

@@ -1799,7 +1799,7 @@ void SwiftEditorDocument::readSyntaxInfo(EditorConsumer &Consumer) {
     llvm::raw_string_ostream OS(SyntaxContent);
     json::Output JsonOut(OS);
     auto Root = Impl.SyntaxInfo->getSourceFile().getSyntaxRoot().getRaw();
-    JsonOut << Root;
+    JsonOut << *Root;
     Consumer.handleSerializedSyntaxTree(OS.str());
   }