Null activity pulled by >1 output caused assert(outLink) error

A NULL activity can arise from code generator optimization but also at runtime when the graph is initialized, if Thor detects that certain activity types (filter type) are not going to produce any output (e.g. !helper->canMatchAny()).

The assert was hit, if there were >1 activity pulling from the same activity (a splitter) and both optimized the input away. The implicit null activity only has one pseudo output, but the graph was asking for output indexes >1 and returning null.