Now that destructuring is always represented using List nodes,
make sure that Array nodes can no longer contain null elements,
so well-typed code doesn't have to deal with them unnecessarily.
If an array does contain empty elements, these are now result in
an error and are represented as a ArrayItem with Error value if
error recovery is used.
The implementation is a bit tricky because at the time the Array
node is created, we cannot tell whether it will be used in a
creation or destructuring context. For this reason the error
reporting is delayed parsing has finished.
Closes#876.
