Media Grid: expose the `$.Deffered()` object in `media.view.AttachmentsBrowser.updateContent()` so the grid can call `Backbone.history.start()` when models actually exist. 

This is a lot better then `_.delay( this.pray, 1000 )`, but needs continued study.

See #29052.

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After [29339] and [29341], add more unit tests and less ambiguous type-checking before bailing in meta-related functions that expect a numeric value for `$object_id`.

Props SergeyBiryukov.
Fixes #28315.

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