Modernize remaining uses of 0/NULL instead of nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

modules/websocket/editor_debugger_server_websocket.cpp

@@ -87,6 +87,6 @@ EditorDebuggerServerWebSocket::~EditorDebuggerServerWebSocket() {
 }
 
 EditorDebuggerServer *EditorDebuggerServerWebSocket::create(const String &p_protocol) {
-	ERR_FAIL_COND_V(p_protocol != "ws://", NULL);
+	ERR_FAIL_COND_V(p_protocol != "ws://", nullptr);
 	return memnew(EditorDebuggerServerWebSocket);
 }