RUSTSEC-2026-0184: Potential undefined behavior with Signature from a buffer-created BlameHunk #202

When a Blame is created via Blame::blame_buffer(), and a BlameHunk is retrieved, the pointers to the original author, original committer, final author, and final committer may be null if unavailable. The corresponding BlameHunk methods then create Signatures based on null pointers; attempting to access the data of the Signatures leads to dereferencing null pointers.