Fix BitArray#toggle when toggling empty subrange

[HertzDevil]

When #toggle is called on a BitArray with an empty subrange and a start index that is a multiple of 32, the method incorrectly toggles the entire UInt32s at and before the given start index:

x = BitArray.new(70)
x.to_slice # => Bytes[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
x.toggle(64, 0)
x.to_slice # => Bytes[0, 0, 0, 0, 255, 255, 255, 255, 255]
# note that `x` also has its unused bits set

This PR fixes it by adding an early return for count == 0.