UI: Don't try to load replay buffer hotkey if null

If replay buffer hotkey isn't set, this would cause an error to be
logged due to a null string.

UI/window-basic-main-outputs.cpp

@@ -367,9 +367,14 @@ SimpleOutput::SimpleOutput(OBSBasic *main_) : BasicOutputHandler(main_)
 		bool useReplayBuffer = config_get_bool(main->Config(),
 						       "SimpleOutput", "RecRB");
 		if (useReplayBuffer) {
+			obs_data_t *hotkey;
 			const char *str = config_get_string(
 				main->Config(), "Hotkeys", "ReplayBuffer");
-			obs_data_t *hotkey = obs_data_create_from_json(str);
+			if (str)
+				hotkey = obs_data_create_from_json(str);
+			else
+				hotkey = nullptr;
+
 			replayBuffer = obs_output_create("replay_buffer",
 							 Str("ReplayBuffer"),
 							 nullptr, hotkey);