tree-wide: use sd_id128_is_null() instead of sd_id128_equal where appropriate

It's a bit easier to read because shorter. Also, most likely a tiny bit faster.

src/journal/journalctl.c

@@ -1266,7 +1266,7 @@ static int add_boot(sd_journal *j) {
         /* Take a shortcut and use the current boot_id, which we can do very quickly.
          * We can do this only when we logs are coming from the current machine,
          * so take the slow path if log location is specified. */
-        if (arg_boot_offset == 0 && sd_id128_equal(arg_boot_id, SD_ID128_NULL) &&
+        if (arg_boot_offset == 0 && sd_id128_is_null(arg_boot_id) &&
             !arg_directory && !arg_file)
 
                 return add_match_this_boot(j, arg_machine);

src/journal/journald-server.c

@@ -877,7 +877,7 @@ void server_driver_message(Server *s, sd_id128_t message_id, const char *format,
         assert_cc(6 == LOG_INFO);
         IOVEC_SET_STRING(iovec[n++], "PRIORITY=6");
 
-        if (!sd_id128_equal(message_id, SD_ID128_NULL)) {
+        if (!sd_id128_is_null(message_id)) {
                 snprintf(mid, sizeof(mid), LOG_MESSAGE_ID(message_id));
                 IOVEC_SET_STRING(iovec[n++], mid);
         }