Fix warning complaining about missing resource with resource list is empty.

CMake/ctkMacroBuildLib.cmake

@@ -129,7 +129,7 @@ macro(ctkMacroBuildLib)
   else()
     QT4_WRAP_UI(MY_UI_CPP ${MY_UI_FORMS})
   endif()
-  if(DEFINED MY_RESOURCES)
+  if(DEFINED MY_RESOURCES AND NOT MY_RESOURCES STREQUAL "")
     if(CTK_QT_VERSION VERSION_GREATER "4")
       qt5_add_resources(MY_QRC_SRCS ${MY_RESOURCES})
     else()