Don't assume a tag has a tagger

Not all tags need to have a tagger. One real-world example can be found in git itself.

```
jacques@Jacquess-MacBook-Pro:~/github/p5-Git-Raw/git $ git cat-file tag v0.99
object a3eb250f996bf5e12376ec88622c4ccaabf20ea8
type commit
tag v0.99

Test-release for wider distribution.

I'll make the first public RPM's etc, thus the tag.

jacques@Jacquess-MacBook-Pro:~/github/p5-Git-Raw/git $ git cat-file tag v0.99.1
object 78d9d414123ad6f4f522ffecbcd9e4a7562948fd
type commit
tag v0.99.1
tagger Linus Torvalds <torvalds@g5.osdl.org> 1121468952 -0700

```