Merge branch 'tb/ref-filter-empty-modifier' into maint

In the "--format=..." option of the "git for-each-ref" command (and
its friends, i.e. the listing mode of "git branch/tag"), "%(atom:)"
(e.g. "%(refname:)", "%(body:)" used to error out.  Instead, treat
them as if the colon and an empty string that follows it were not
there.

* tb/ref-filter-empty-modifier:
  ref-filter.c: pass empty-string as NULL to atom parsers