macosx: enter fullscreen only if we were on Lion or we got an actual video to show (fixes #5611)

this commit also fixes a xib incompatiblity with 10.5

modules/gui/macosx/MainWindow.m

@@ -913,10 +913,6 @@ static VLCMainWindow *_o_sharedInstance = nil;
         else
             [o_bottombar_view setHidden: NO];
     }
-    else
-    {
-        [o_fullscreen_btn setEnabled: b_videoPlayback];
-    }
 }
 
 - (void)resizeWindow

modules/gui/macosx/intf.m

@@ -1342,7 +1342,7 @@ unsigned int CocoaKeyToVLC( unichar i_key )
     {
         input_thread_t * p_input = pl_CurrentInput( VLCIntf );
 
-        if( p_input != NULL )
+        if( p_input != NULL && [self activeVideoPlayback])
         {
             if(b_fullscreen)
                 [o_mainwindow performSelectorOnMainThread:@selector(enterFullscreen) withObject:nil waitUntilDone:NO];