sd-id128: return -ENOMEDIUM on null id

We currently return -ENOMEDIUM when /etc/machine-id is empty, and -EINVAL when
it is all zeros. But -EINVAL is also used for invalid args. The distinction
between empty and all-zero is not very important, let's use the same return
code.

Also document -ENOENT and -ENOMEDIUM since they can be a bit surprising.

src/libsystemd/sd-id128/sd-id128.c

@@ -98,7 +98,7 @@ _public_ int sd_id128_get_machine(sd_id128_t *ret) {
                         return r;
 
                 if (sd_id128_is_null(saved_machine_id))
-                        return -EINVAL;
+                        return -ENOMEDIUM;
         }
 
         *ret = saved_machine_id;