Add test for HashMap::get_mut with non-existent key

[cppbear]

Hi team,

This pull request adds a test aimed at improving the test coverage within the repository.

The primary focus of these tests is on the HashMap::get_mut. We've ensured these tests are robust and cover key scenarios.

map.rs

hashbrown/src/map.rs

Line 1452 in 6263458

None => None,

hashbrown/src/map.rs

Line 1462 in 6263458

None

raw/mod.rs

hashbrown/src/raw/mod.rs

Line 1868 in 6263458

probe_seq.move_next(self.bucket_mask);

We appreciate your time in reviewing these additions.

Thanks.

[cppbear]

Hello! I was wondering if there's any feedback on this PR. I've ensured all checks are passing and I'm happy to address any concerns or make changes as needed. Thank you!