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/machine/machine.c

@@ -181,7 +181,7 @@ int machine_save(Machine *m) {
                 fprintf(f, "ROOT=%s\n", escaped);
         }
 
-        if (!sd_id128_equal(m->id, SD_ID128_NULL))
+        if (!sd_id128_is_null(m->id))
                 fprintf(f, "ID=" SD_ID128_FORMAT_STR "\n", SD_ID128_FORMAT_VAL(m->id));
 
         if (m->leader != 0)

src/machine/machinectl.c

@@ -528,7 +528,7 @@ static void print_machine_status_info(sd_bus *bus, MachineStatusInfo *i) {
 
         fputs(strna(i->name), stdout);
 
-        if (!sd_id128_equal(i->id, SD_ID128_NULL))
+        if (!sd_id128_is_null(i->id))
                 printf("(" SD_ID128_FORMAT_STR ")\n", SD_ID128_FORMAT_VAL(i->id));
         else
                 putchar('\n');