Fixes cast conformance tests

Description

Fixes cast conformance tests.

We have decided that NULL and MISSING aren't types. They are values that may fit into the domains of all other types.
- Instead of CAST(<expr> AS NULL), one should just use NULL. Same goes for MISSING.
Fixes the CAST(<expr> AS INT) tests.
- INT corresponds with a 4-byte integer (range of: -2147483648 to 2147483647)
- BIGINT corresponds with an 8-byte integer (range of: -9223372036854775808 to 9223372036854775807)
- The old tests cast BIGINT values to INT (causing failures). These tests are wrong, and I've updated them to reflect the expected behavior.

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