Consume cached nil in Unpacker#skip_nil

[OskarEichler]

Summary

Consume the cached nil marker when Unpacker#skip_nil succeeds. The current native implementation returns true without resetting the cached head byte, so the next read returns the same nil instead of the following object.

Reproduction

Feeding packed nil followed by packed 1, then calling skip_nil and read, returns nil on current master. This branch returns 1.

Verification

• 458 examples, 0 failures, 1 existing pending example
• focused before/after model on source and the freshly installed native gem
• MRI native build, Ruby syntax, exact package/install, Rails 8.1.3.1 boot, and Bootsnap round-trip

The source change and this contribution were prepared with AI assistance and manually reviewed.