Improve `WP_Tax_Query` param sanitization for empty strings.

When an empty string is passed as one of the clauses in the `$tax_query`
parameter, it should be discarded rather than parsed as a first-order clause.

Props tmtrademark.
Fixes #30117.

git-svn-id: https://develop.svn.wordpress.org/trunk@30031 602fd350-edb4-49c9-b593-d223f7449a82