Create a NULL model definition. This model produces NA forecasts and does not
require any estimation of the data. It is generally used as a placeholder for
models which have encountered an error (see `.safely`

in `model()`

).

```
null_model(formula, ...)
is_null_model(x)
```

## Arguments

- formula
Model specification (response variable)

- ...
Unused

- x
The object to be tested.