Return nil if count is 0.

This fixes an issue where `RACIndexSetSequence`
would return data from `self.indexes` in `head`
even if it represented an empty index set.
It now returns `nil` if it represents an empty
set.
Fix bug in NSIndexSet tests.

The test for NSIndexSet+RACSupport includes a
block that returns an array of all values in an
index set. This block mistakenly referenced a
captured index set declared outside of itself
instead of referring to its sole parameter.
Add a test for empty index sets.
Better solution to the empty index set problem.

Return `[self empty]` early if the index set
contains no indexes.
Code style.
Merge pull request #1203 from calebd/bug-fix-nsindexset

Return nil if count is 0 in RACIndexSetSequence