Fixed lack of bounds checking for objectAtIndexedSubscript on PMKArray

Not sure if this was intended or not, but if you return a lower amount of arguments via PMKManifold than the block expects, it tries to access these elements anyway.

This fix ensures that any attempt to access beyond the bounds of the array returns nil, which allows for the resulting block to evaluate with a subset of the arguments it expects. 

This is mainly useful in a case where the most important parameter is present, but the other parameters aren't always present or necessary to pass back - which is my current use case.