Potential undefined behavior with Signature from a buffer-created BlameHunk

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 `Signature`s based on null pointers; attempting to access the data of the `Signature`s leads to dereferencing null pointers.