kernel: Fix IndexOutOfRange with empty matches in the regexp

It is possible to have a non nil Interval that is empty. Make
the code work on isNil or isEmpty to avoid the exception It
might be that the Intervals of registers might never be nil.

Fixes:
SystemExceptions.IndexOutOfRange(Exception)>>signal (ExcHandling.st:254)
SystemExceptions.IndexOutOfRange class>>signalOn:withIndex: (SysExcept.st:660)
Interval>>first (Interval.st:245)
Kernel.MatchingRegexResults>>at: (Regex.st:382)

2013-12-08  Holger Hans Peter Freyther  <holger@freyther.de>

	* kernel/Regex.st: Check for isEmpty of the Interval before
	trying to use it.

2013-12-08  Holger Hans Peter Freyther <holger@freyther.de>

	* kernel/RegexpTests.st: Add tests for Regexp.