Convert null_model to be simply NULL

[mitchelloharawild]

There are many cases in which NULL can be generated from other tidyverse tools.
Handling NULL models should be handled with the simpler NULL value.

[edgBR]

This will be a very nice feature.

It will be much easier to clean the mable.

BR
/E

[mitchelloharawild]

This is slightly challenging as currently the NULL model stores its own response variables for formatting fitted()/residuals() output. As a mable can only contain a single response variable, this metadata should be brought up to the mable level and not be stored within models.

[mitchelloharawild]

MRE:

library(fable)
#> Loading required package: fabletools
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
lung_deaths <- as_tsibble(cbind(mdeaths, fdeaths))
mbl <- lung_deaths %>% model(a = TSLM(value ~ season()), b = SNAIVE(value))
bind_rows(mbl %>% slice(1), mbl %>% slice(2) %>% select(a))
#> Error in UseMethod("response_vars"): no applicable method for 'response_vars' applied to an object of class "NULL"

^{Created on 2021-01-08 by the reprex package (v0.3.0)}