fix zero descriptor initialization

src/common/descriptor.c

@@ -42,7 +42,7 @@ static knot_lookup_table_t dns_classes[] = {
  */
 static const rdata_descriptor_t rdata_descriptors[] = {
 	[0]                      = { { KNOT_RDATA_WF_REMAINDER,
-	                               KNOT_RDATA_WF_END }, "" },
+	                               KNOT_RDATA_WF_END }, NULL },
 	[KNOT_RRTYPE_A]          = { { 4, KNOT_RDATA_WF_END }, "A" },
 	[KNOT_RRTYPE_NS]         = { { KNOT_RDATA_WF_COMPRESSED_DNAME,
 	                               KNOT_RDATA_WF_END }, "NS" },