Meta boxes: Don't assume that callback args are an array.

While the documentation for add_meta_box() specifices that $callback_args should be an array, this has never been enforced, and we have workarounds in place for when it's passed as something other than an array.

Rather than break sites that are passing unexpected data, we can quietly just allow for it, instead.

Props johnjamesjacoby, birgire, pento.
Merges [43838] to trunk.
Fixes #45206.

Built from https://develop.svn.wordpress.org/trunk@44174


git-svn-id: https://core.svn.wordpress.org/trunk@44004 1a063a9b-81f0-0310-95a4-ce76da25c4cd