It should be possible to set `$quantiles` to `NULL`

RegrLearner's `$quantiles` are initialized as `NULL`, so I think it should be possible to reset the value to NULL agian.

```r
ld <- lrn("regr.debug")
ld$quantiles
#> NULL
ld$quantiles <- .5
ld$quantiles <- NULL
#> Error in .__LearnerRegr__quantiles(self = self, private = private, super = super,  : 
#>   Assertion on 'quantiles' failed: Must be of type 'numeric', not 'NULL'.
```