Undefined global variables evaluate to MISSING instead of giving an error.

[dlurton]

I don't think this is a strict vs permissive mode thing at all: I think this should probably be an error regardless of typing mode.

Repro case, paste this in side of a #[cfg(test)] in partiql-logical-planner/src/lib.rs:

    #[test]
    pub fn test_foo() {
        let out = evaluate_query("undefined_global_variable");
        println!("{:?}", &out);
    }