ListTags: Fix NPE when annotated/signed tag has no tagger info

The implementation assumed that annotated/signed tags would always
have a tagger, but this is not the case as we can see with the tag
v0.99 on the git project [1], and this causes a NullPointerException
when trying to convert to a GitPerson.

Add a null check.  Also update the documentation accordingly.

[1] http://git.kernel.org/cgit/git/git.git/tag/?h=v0.99

Change-Id: I7dcb98a845d9a9e83c60d32f7ae5b51eba38bab6
2 files changed