Notice in `WP_Tax_Query::get_sql_for_clause()`

[dlh]

WP_Tax_Query::get_sql_for_clause() provides a default empty string for the $join variable, but not for $where.

This can lead to a undefined-variable notice from $sql['where'][] = $where if the received $clause['operator'] isn't covered in the method's conditionals.

The attached patch would create $where with $join.

[boonebgorges]

Thanks for the ticket!

[boonebgorges]

In 32910:

Fix PHP notice in WP_Tax_Query::get_sql_for_clause().

Previously, the $where variable was not initialized, so that when an invalid
'operator' was passed, PHP would complain that $where was undefined.

Props dlh.
Fixes #32756.