Fix a rare edge-case bug where invalid values in tax_query terms resulted in a broken query

[rinatkhaziev]

Description

This is just a quick copy of 10up#2576

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).
• This change has the fix PRed upstream (if applicable). If not applicable, it has the relevant "// VIP: reason for the discrepancy with upstream" comment in places where the code is discrepant.

Steps to Test

[jesse-greathouse]

Thanks for fixing this.