PHP Notice if nothing selected on custom bulk action in Media Library.

[gitlost]

If you add a custom bulk action to the Media Library:

<?php
add_filter( 'bulk_actions-upload', function( $actions ) {
        $actions['foo'] = 'Foo';
        return $actions;
} );

Then if nothing selected when you apply "Foo" (and you have WP_DEBUG on) you'll get a

PHP Notice:  Undefined variable: post_ids in .../wp-admin/upload.php on line 168

This is a first pass at fixing this. This version does change behavior in two key ways.

1) It always runs the filter, even for those cases where there is nothing selected. In this case, it passes an empty array for $post_ids. This allows the filter to put up an appropriate error message.

2) It also checks the filter return value. If the filter returns false, null, or empty string, it will use the previously defined $location instead.

This behavior change may be unexpected to some. We should discuss these changes to make sure these are appropriate.

Looking at [38647] none of the other handle_bulk_actions filters checks the return so perhaps it's best to leave it as is and just do the isset() check (which should be before the comment). Also I don't think it constitutes a change of behaviour as it always runs now - adding the check just saves one having to do something similar in the handler...

Revised based on feedback

Patch is revised to not check the return value, but blindly assign to $location, matching behavior of non-filtered handlers.