IcingaObject: Allow replaceWith on non-apply Objects

This broke with assign_filter changes, we should not throw an error when
the filter is actually null.

replaceWith() re-sets all vars.

refs #12033

library/Director/Objects/IcingaObject.php

@@ -396,7 +396,7 @@ abstract class IcingaObject extends DbObject implements IcingaConfigRenderer
      */
     public function setAssign_filter($filter)
     {
-        if (! $this->supportsAssignments()) {
+        if (! $this->supportsAssignments() && $filter !== null) {
             if ($this->hasProperty('object_type')) {
                 $type = $this->object_type;
             } else {