empty node type for easy composition

I often find myself wanting to conditionally add a node to a list when building an AST. For example when building the argument list for a `b.callExpression`. Currently the way to do it is to pull out the args array and conditionally push something to it and then pass it to the builder function. It'd be nice to have an empty node type that recast can ignore.

```
b.callExpression(
fun,
[
a,
b,
condition ? b.literal(c) : b.empty()
]
);
```

This will also make `.map`ing over nodes much easier.

Start by tracing the builder entry points shown in the example, especially b.callExpression and the proposed b.empty(), and inspect how recast processes nodes in lists. Done means an empty node can be returned from conditional expressions or mapped lists and is ignored during output without requiring manual array edits.

Written by the indexing model from the issue text.