ref-filter.c: pass empty-string as NULL to atom parsers
Peff points out that different atom parsers handle the empty
"sub-argument" list differently. An example of this is the format
"%(refname:)".
Since callers often use `string_list_split` (which splits the empty
string with any delimiter as a 1-ary string_list containing the empty
string), this makes handling empty sub-argument strings non-ergonomic.
Let's fix this by declaring that atom parser implementations must
not care about distinguishing between the empty string "%(refname:)"
and no sub-arguments "%(refname)". Current code aborts, either with
"unrecognised arg" (e.g. "refname:") or "does not take args"
(e.g. "body:") as an error message.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>