WPDB: Force `STRICT_ALL_TABLES` to be enabled as soon as we connect to the MySQL server.

This improves data integrity when inserting and updating rows in the database, particularly when trying to insert emoji into posts stored with character sets that don't support emoji.

See #21212.

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Return an empty array from `get_approved_comments()` when `$post_id` is empty.

This behavior was broken when moving the internals to `WP_Comment_Query` in
[30098]. As a result, `get_approved_comments( 0 )` was fetching *all* approved
comments, causing performance issues.

Props dd32.
Fixes #30412.
Built from https://develop.svn.wordpress.org/trunk@30402


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