arrow

ARROW-7064: [R] Support null type using vctrs::unspecified()

Not sure `vctrs::unspecified()` is the right concept, but e.g.

``` r
library(arrow)
#>
#> Attaching package: 'arrow'
#> The following object is masked from 'package:utils':
#>
#>     timestamp

a <- Array$create(vctrs::unspecified(10))
a
#> Array
#> <null>
#> 10 nulls
a$type
#> Null
#> null
a$as_vector()
#> <unspecified> [10]

b <- record_batch(a = 1:10, n = vctrs::unspecified(10))
b$schema
#> Schema
#> a: int32
#> n: null
```

<sup>Created on 2019-12-12 by the [reprex package](https://reprex.tidyverse.org) (v0.3.0.9000)</sup>

Closes #6023 from romainfrancois/ARROW-7064/null_type and squashes the following commits:

b275d90b8 <Romain Francois> rename Converter_Unspecified to Converter_Null
d9bfd4d01 <Romain Francois> Support null type using vctrs::unspecified()

Authored-by: Romain Francois <romain@rstudio.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>