UI: Fix YouTubeAppDock crash

Apparently the YouTube app dock thing didn't take into consideration the
fact that the user may have the browser source available, but may not
have browser docks available (via the `cef` global). This is because
certain system configurations do not support browser widgets, such as
Wayland on Linux.

(Lain note: There's multiple things I'm unhappy about here. I'm mostly
going to blame myself for multiple reasons.)

UI/window-basic-main.cpp

@@ -8493,6 +8493,9 @@ YouTubeAppDock *OBSBasic::GetYouTubeAppDock()
 
 void OBSBasic::NewYouTubeAppDock()
 {
+	if (!cef)
+		return;
+
 	if (youtubeAppDock)
 		delete youtubeAppDock;
 	youtubeAppDock = new YouTubeAppDock();
@@ -8500,6 +8503,9 @@ void OBSBasic::NewYouTubeAppDock()
 
 void OBSBasic::DeleteYouTubeAppDock()
 {
+	if (!cef)
+		return;
+
 	if (youtubeAppDock)
 		delete youtubeAppDock;
 	youtubeAppDock = nullptr;