Tagger can be empty, as can Commit and Author - tolerate this (#15835)

Tagger can be empty, as can Commit and Author - tolerate this (#15835)

Unfortunately some old repositories can have tags with empty Tagger, Commit
or Author. Go-Git variants will always have empty values for these whereas
the native git variant leaves them at nil. The simplest solution is just to
always have these set to empty Signatures.

v156 migration also makes the incorrect assumption that these cannot be empty.
Therefore add some handling to this and add logging and adjust broken
logging elsewhere in this migration.

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>