bugfix: change LazyOffsetArrayIter16 to ignore null offsets

[asibahi]

this changes the next function in LazyOffsetArrayIter16to ignore None values in the array. All tests pass in tif-parser and rustybuzz. no code changes required in rustybuzz.

closes #211 , as I noticed none of call sites in rustybuzz expect a non-fused iterator. (they're mostly collect and any calls).

Edit:

Looking at the implementation of the corresponding data structure in read-fonts, it does not skip over the null offsets. However, all usage I could find map or filter_map except in a test).