[generator][trace] LeafIterator cannot handle zero length TraceRegions

using xtext 2.8.4

the LeafIterator.computeNext() method may run into a infinite loop if there is a trace region with length zero.

if current.offset == expectedOffset it will return firstLeafOfCurrent will be returned.
this does not advance expectedOffset since the length is zero.

i assume the zero length based regions should be filtered out before (or never been created) or the iterator should be able to handle them.

Start with LeafIterator.computeNext() and trace-region handling in the generator trace subsystem. Reproduce the infinite loop using a TraceRegion with zero length, then determine whether such regions should be filtered before iteration or handled by the iterator. Done means zero-length regions no longer cause non-termination.