#7944 fix null-id filter crash

Hello, here's the source issue: #7944 and my comment on it. Original author have closed the issue but the crash remain. So, here's my fix.

In short, if an element inside <defs> doesn't have an id attribute, then internal filter effects will crash the app due to null-dereference.

I don't know whether I should make the same fix for 1.2.x branch and any other.

Thanks.