Do not allow WHERE clause terms to match constant string index terms, which

can happen if DQS_DDL is enabled.  Follow-up to
[44200596aa943963].  dbsqlfuzz 54c9db85ed4af7055f5fd0d50877875c82b11d46.

FossilOrigin-Name: 2d2b91cc0f6fed8cb6f738dc7019047ce0f1e86b5eb8efa997095d08a32cbcb6