Similarly to #1554142: drupal_array_merge_deep_array assumes $array is actually an array, I would think this was the caller's responsibility. The function is documented as taking an array, and doesn't not provide any defaults, so it shouldn't need to check for an array.
New patch attached
drupal_group-jss is defined at http://drupalcode.org/project/drupal.git/blob/refs/heads/8.x:/core/modul...
drupal_get_js returns a render array with #items set to at least an empty array, which is passed to drupal_group_js. If the array is empty, the foreach is smart enough to not loop through it.
The only reason this would be needed would be if its not an array at all, which would mean a bug elsewhere.
@Tim drupal_get_js can also return an empty string:
AFAICS, that return value should be one of the #items, not the only one.
'#type' => 'script','#items' => array( // each entry in here is the result of drupal_get_js, so it can be an array or '' // but #items itself is what drupal_group_js operates on and it should always be an array),
The issue is solved another way for D8, relevant if we want to fix the other issue for D7.
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