JIT: allow nulls in gtCanOptimizeTypeEquality (#13680)

JIT: allow nulls in gtCanOptimizeTypeEquality (#13680)

This is a follow-on to #13657. I looked at the remaining calls to
`Type::op_Equality` in the jit-diffs output and saw many of the calls had a
null pointer argument. This pattern comes about from explicit null checks in
the sources, often as part of argument validation.

Such calls can also be optimized into simple pointer equality checks, so
add another clause to `gtCanOptimizeTypeEquality` to look for nulls.