[ty] Fix match exhaustiveness for enum | None unions (#22290)
## Summary
If we match on an `TestEnum | None`, then when adding a case like
`~Literal[TestEnum.FOO]` (i.e., after `if value == TestEnum.FOO:
return`), we'd distribute `Literal[TestEnum.BAR]` on the entire builder,
creating `None & Literal[TestEnum.BAR]` which simplified to `Never`.
Instead, we should only expand to the remaining members for pieces of
the intersection that contain the enum.
Now, `(TestEnum | None) & ~Literal[TestEnum.FOO] &
~Literal[TestEnum.BAR]` correctly simplifies to `None` instead of
`Never`.
Closes https://github.com/astral-sh/ty/issues/2260.
