Fix GROUP BY when value is MISSING

[alancai98]

(Related to GROUP BY test failures) Current implementation of GROUP BY has the correct behavior for the case when creating groups for a value that's NULL in one binding tuple and MISSING in another (as defined in section 11.1.1). But for the edge case of when the value is just MISSING, the group uses NULL as the tuple value rather than MISSING.

[alancai98]

The current behavior in partiql-lang-rust is correct. That is, a group key of MISSING should coerce to NULL even when all the values are MISSING. The conformance tests that were ported from Kotlin need to be fixed for this case.

Tracking issue for spec: partiql/partiql-lang#15.
Tracking issue for conformance test fix: partiql/partiql-tests#73.