list-objects-filter: use empty string instead of NULL for sparse "base"

We use add_excludes_from_blob_to_list() to parse a sparse blob. Since
we don't have a base path, we pass NULL and 0 for the base and baselen,
respectively. But the rest of the exclude code passes a literal empty
string instead of NULL for this case. And indeed, we eventually end up
with match_pathname() calling fspathncmp(), which then calls the system
strncmp(path, base, baselen).

This works on many platforms, which notice that baselen is 0 and do not
look at the bytes of "base" at all. But it does violate the C standard,
and building with SANITIZE=undefined will complain. You can also see it
by instrumenting fspathncmp like this:

	diff --git a/dir.c b/dir.c
	index d021c908e5..4bb3d3ec96 100644
	--- a/dir.c
	+++ b/dir.c
	@@ -71,6 +71,8 @@ int fspathcmp(const char *a, const char *b)

	 int fspathncmp(const char *a, const char *b, size_t count)
	 {
	+	if (!a || !b)
	+		BUG("null fspathncmp arguments");
	 	return ignore_case ? strncasecmp(a, b, count) : strncmp(a, b, count);
	 }

We could perhaps be more defensive in match_pathname(), but even if we
did so, it makes sense for this code to match the rest of the exclude
callers.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

list-objects-filter.c

@@ -472,7 +472,7 @@ static void *filter_sparse_oid__init(
 		die(_("unable to access sparse blob in '%s'"),
 		    filter_options->sparse_oid_name);
 	d->omits = omitted;
-	if (add_excludes_from_blob_to_list(&sparse_oid, NULL, 0, &d->el) < 0)
+	if (add_excludes_from_blob_to_list(&sparse_oid, "", 0, &d->el) < 0)
 		die(_("unable to parse sparse filter data in %s"),
 		    oid_to_hex(&sparse_oid));