0-ary lambda

In MathML2 (4.2.1.7), lambda can explicitly be used with zero bvar
elements to construct a 0-ary function (like random() or timerightnow()).

The current spec of MathML3 requires at least one bvar child for lambda
- this needs to be fixed in a number of places.

This means that the transformation rule for lambda to "strict" MathML is
broken as-is in the case of 0-ary lambda, as there is no bvar in the
resulting apply to trigger binding.