Further simplification of untagged variant output

[cknitt]

Followup to #7141.

https://github.com/rescript-lang/rescript-compiler/blob/43d2bbf33fd0d657591aeea719b35a1c316ed77b/tests/tests/src/core/Core_NullableTests.mjs#L10

Could be simplified to tNull === null.

https://github.com/rescript-lang/rescript-compiler/blob/43d2bbf33fd0d657591aeea719b35a1c316ed77b/tests/tests/src/core/Core_NullableTests.mjs#L21

Could be simplified to tUndefined === undefined.

https://github.com/rescript-lang/rescript-compiler/blob/43d2bbf33fd0d657591aeea719b35a1c316ed77b/tests/tests/src/core/Core_NullableTests.mjs#L32

Could be simplified to tValue === "hello".

[cristianoc]

And this:
#7135 (review)