OS X: Fix app bundle search path during cross compiling (#14603)

In "Modules/Platform/Darwin.cmake" the variable _apps_paths stays empty
if cross compiling.  Do not de-duplicate an empty list.

Modules/Platform/Darwin.cmake

@@ -341,7 +341,9 @@ foreach(_path
     list(APPEND _apps_paths "${_apps}")
   endif()
 endforeach()
-list(REMOVE_DUPLICATES _apps_paths)
+if(_apps_paths)
+  list(REMOVE_DUPLICATES _apps_paths)
+endif()
 set(CMAKE_SYSTEM_APPBUNDLE_PATH
   ${_apps_paths})
 unset(_apps_paths)