Merge branch 'mp/for-each-ref-missing-name-or-email'

"for-each-ref" and friends that shows refs did not protect themselves
against ancient tags that did not record tagger names when asked to
show "%(taggername)", which have been corrected.

* mp/for-each-ref-missing-name-or-email:
ref-filter: initialize empty name or email fields

diff --cc ref-filter.c
Simple merge

diff --cc t/t6300-for-each-ref.sh
Simple merge