wp_list_pages() puts leading comma in exclude string

[kevinB]

When called with no 'exclude' argument, wp_list_pages() passes an array with a nullstring element to the filter 'list_pages_excludes'. If a plugin then adds at least one more element to that array, the result is a leading comma in the 'exclude' argument string which is passed to get_pages() and walk_page_tree().

This misplaced comma does not affect WP core behavior. But if a plugin which filters 'get_pages' copies the 'exclude' argument to 'exclude_tree' (logical enough if depth=1 is also passed) and mimics core handling of that argument, the result is exclusion of the Main Page (ID zero) and all its children.

[kevinB]

wp_list_pages(): prevent leading comma in exclude argument

[markjaquith]

(In [12327]) Get rid of stray comma. fixes #10977. props kevinB