Widgets: Fix previewing embeds in Text widget by allowing `parse-embed` admin...

Widgets: Fix previewing embeds in Text widget by allowing `parse-embed` admin ajax requests with an empty `post_ID` just as `WP_oEmbed_Controller::get_proxy_item_permissions_check()` allows.

As of #34115 if there is no post context the oEmbed will be cached in an `oembed_cache` custom post type, so having a post as context is no longer a requirement for caching.

Props biskobe, westonruter.
See #34115, #40450.
Fixes #40854.

Built from https://develop.svn.wordpress.org/trunk@41913


git-svn-id: https://core.svn.wordpress.org/trunk@41747 1a063a9b-81f0-0310-95a4-ce76da25c4cd