#28 Support pattern-matching against NULL child pointers

[Tom Harwood]

Allow the pattern-matcher to match nodes with NULL child pointers. Needs some thought; the dodge of just skipping NULL children is easy to implement, but experience showed the pattern matchers were so confusing it was more maintainable to adjust the input.

If NULL children were a bona fide part of the pattern, it would be much more maintainable, but the annotation selector would have to do an initial phase to find nodes with NULL children and without. They would in effect be two different types of node.