Make a dedicated `EmptyNode`

[tpgillam]

This would be similar to a Constant, but even simpler as it doesn't have a value — just a value_type.

We should then:

• Alter empty_node to return something of this type
• Add _is_empty(::Node), similar to _is_constant
• Add optimisations and test for them — in many cases, applying a function to an empty node will be a no-op.