wp_generate_attachment_metadata() should not call WP_Image_Editor if there's nothing to resize

[mdawaffe]

wp_generate_attachment_metadata() calls WP_Image_Editor::multi_resize() to create the intermediate sizes of the image.

If there are no sizes to create, WP_Image_Editor::get_instance() should not be called. It will not be used for anything and is a waste of resources.

If it is called, and the image is very large, a needless Out of Memory Fatal Error can result.

Patch attached.

[mdawaffe]

I put in the else condition to set $metadata['sizes'] = array() since that's what the current behavior in trunk is: ::multi_resize() returns an empty array if $sizes is empty.

In 3.4, $metadata['sizes'] would have been left unset.

[nacin]

In 22571:

wp_generate_attachment_metadata() should not call WP_Image_Editor if there's nothing to resize. props mdawaffe. fixes #22438.