When using RNA to alter the type of socket only the type was changed.
the typeinfo was not updated. Internally the File Output Node used RNA
to update the sockettype. making the socket invalid. When users save the
file and reopened the typeinfo was used. Also the color of the node was
determined via the typeinfo.
Another thing that happened was that the socket conversion was ignored when empty node groups were present. The empty node groups were optimized away before the needed data conversion was determined.