WPDB: Check that `AUTH_SALT` is not empty.

In `wpdb::placeholder_escape()`, the key for `hash_hmac()` defaults to `AUTH_SALT`, but `hash_hmac()` will return an empty string if the key is empty.

This had the side effect of the string `{}` being incorrectly replaced with a `%` character in queries just about to be run on the database.

Props jsonfry.
Fixes #42431.


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git-svn-id: https://core.svn.wordpress.org/trunk@41949 1a063a9b-81f0-0310-95a4-ce76da25c4cd