obs-frontend-api: Make a few frontend API thread-safe

Additionally, check for null output handlers.

UI/window-basic-main.cpp

@@ -6998,3 +6998,24 @@ OBSBasic *OBSBasic::Get()
 {
 	return reinterpret_cast<OBSBasic*>(App()->GetMainWindow());
 }
+
+bool OBSBasic::StreamingActive()
+{
+	if (!outputHandler)
+		return false;
+	return outputHandler->StreamingActive();
+}
+
+bool OBSBasic::RecordingActive()
+{
+	if (!outputHandler)
+		return false;
+	return outputHandler->RecordingActive();
+}
+
+bool OBSBasic::ReplayBufferActive()
+{
+	if (!outputHandler)
+		return false;
+	return outputHandler->ReplayBufferActive();
+}