diff_requirements() should always return with array

As long as diff_requirements() may return with NULL, which will cause drupal_check_profile() get mad with array_merge():

Warning: array_merge(): Argument #2 is not an array in drupal_check_profile() (line 955 of core/includes/install.inc).

drupal_check_profile('drustack') (Line: 1953)
install_check_requirements(Array) (Line: 1021)
install_verify_requirements(Array) (Line: 660)
install_run_task(Array, Array) (Line: 538)
install_run_tasks(Array) (Line: 115)
install_drupal(Object) (Line: 44)

Simply always return $requirement even with empty array can solve the problem.