Make Nulls Terminal

[hudson-ai]

Teeny-tiny little PR: make Null inherit from Terminal.

As far as I can tell, this is only really useful for preventing something rather silly from blowing up:

@guidance
def null_grammar(lm):
  return lm

Why would you write this? Don't ask.

Currently, calling this will raise an AttributeError: 'Null' object has no attribute 'values' when replace_grammar_node does its thing.

An alternative fix would be to explicitly add Null to this isinstance check in replace_grammar_node:

def replace_grammar_node(grammar, target, replacement):
  ...
    # We are done with this node if it's a terminal
    if isinstance(current, (Terminal, ModelVariable)):
            continue

But I think that this PR is a bit more elegant (unless anyone can think of a reason that Null wouldn't be Terminal).

All tests passing on my end :)

[slundberg]

That's a corner case I had not run across! Thanks @hudson-ai looks good to me.