Expected but undefined

I woke up yesterday in a situation of

```json
{
  "result": "{{>result}}"
}
```

The test was passing, although `result` was undefined.

Given that KATT works on partial matches, I think it should actually fail when it should store a value, and that value is undefined.

What do you think, @isakb?

**UPDATE**: added also a check for the situation where "you *expect* a value to be *unexpected* i.e. undefined".