fix matRadGUI return value if GUI is disabled in matRad_Config

Author: wahln

matRadGUI.m

@@ -62,11 +62,10 @@
 
 if matRad_cfg.disableGUI
     matRad_cfg.dispInfo('matRad GUI disabled in matRad_cfg!\n');
+    hGUI = [];
     return;
 end
 
-handleValid = true;
-
 try
     handleValid = ishandle(hMatRadGUI.guiHandle);
 catch