In preparation of the relaxation of the grammar to return any expression,
change the whereClause production to return an expression rather than
terms. Note that the expression is still constrained to be a conjunction
of relations, and our filtering code isn't prepared for more.
Before the patch, if the WHERE clause was optional, the grammar would
pass an empty vector of expressions (which is exactly correct). After
the patch, it would pass a default-constructed expression. Now that
happens to be an empty conjunction, which is exactly what's needed, but
it is too accidental, so the patch changes optional WHERE clauses to
explicitly generate an empty conjunction if the WHERE clause wasn't
specified.
9823e75d16