Fix die that should be return undef

I got ahead of myself -- I intend to replace `return undef` with `die`
in a future commit, and somehow this one jumped the gun.

This fixes a bug introduced in commit
eab72ef6d7

ddclient.in

@@ -2565,7 +2565,7 @@ sub check_value {
     } elsif (!defined($value) && $required) {
         # None of the types have 'undef' as a valid value, so check definedness once here for
         # convenience.
-        die("$type is required\n");
+        return undef;
 
     } elsif ($type eq T_DELAY) {
         $value = interval($value);