list-objects-filter: treat NULL filter_options as "disabled"

In most callers, we have an actual list_objects_filter_options struct,
and if no filtering is desired its "choice" element will be
LOFC_DISABLED. However, some code may have only a pointer to such a
struct which may be NULL (because _their_ callers didn't care about
filtering, either). Rather than forcing them to handle this explicitly
like:

  if (filter_options)
          traverse_commit_list_filtered(filter_options, revs,
	                                show_commit, show_object,
					show_data, NULL);
  else
          traverse_commit_list(revs, show_commit, show_object,
	                             show_data);

let's just treat a NULL filter_options the same as LOFC_DISABLED. We
only need a small change, since that option struct is converted into a
real filter only in the "init" function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter.c

@@ -663,6 +663,9 @@ struct filter *list_objects_filter__init(
 
 	assert((sizeof(s_filters) / sizeof(s_filters[0])) == LOFC__COUNT);
 
+	if (!filter_options)
+		return NULL;
+
 	if (filter_options->choice >= LOFC__COUNT)
 		BUG("invalid list-objects filter choice: %d",
 		    filter_options->choice);