SQL AST: parse expressions with no FROM clause

[hemberger]

Expressions without a FROM clause are valid SQL, and can be properly parsed by allowing QueryScope to take a nullable Table argument.

[hemberger]

Instead of allowing QueryScope to take a nullable Table, I'm now creating an empty Table to signify that there is no FROM clause. We could still go the nullable route if that is preferable, but it gets complicated by a bunch of bookkeeping variables to differentiate between $fromTable === null because we have an empty FROM clause (allowed) vs. because we have some other non-empty FROM clause that we don't support (disallowed). Another option is to make Table non-final and have a few subclasses (which is potentially one way to handle the abstraction for FROM subquery in the future).

[staabm]

thank you