Error in `SubgraphFusion`

If auto optimization is applied to the attached SDFG then an missing attribute error is generated.
After a discussion with @alexnick83 on Mattermost and some investigation I discovered that the transformation implicitly assumes that every map exit node is connected to an accessnode.
However, the attached SDFG violates this assumption because there a map exit is connected to another map exit.

[failing_sdfg.json](https://github.com/spcl/dace/files/14345692/failing_sdfg.json)