Missing values are not handled when converting `DictEncoded` to `PooledArray` via `copy`

When copying a column of type `DictEncoded`, the missing values in it are not handled. The resulting `PooledArray` does not have `missing` in its `pool`. Because of that, if one tries to access the element that is supposed to be `missing`, an `UndefRefError` is raised.

Here is an illustration with an example data file:
[cat_with_missing.feather.zip](https://github.com/JuliaData/Arrow.jl/files/6743124/cat_with_missing.feather.zip)

```julia
using Arrow
tb = Arrow.Table("cat_with_missing.feather")
```
```julia
julia> tb.A
3-element Arrow.DictEncoded{Union{Missing, String}, Int8, Arrow.List{Union{Missing, String}, Int32, Vector{UInt8}}}:
missing
"a"
"b"

julia> tb.A[1]
missing

julia> A = copy(tb.A)
3-element PooledArrays.PooledVector{Union{Missing, String}, Int8, Vector{Int8}}:
#undef
"a"
"b"

julia> A[1]
ERROR: UndefRefError: access to undefined reference
Stacktrace:
[1] getindex(A::PooledArrays.PooledVector{Union{Missing, String}, Int8, Vector{Int8}}, I::Int64)
@ PooledArrays ~/.julia/packages/PooledArrays/CV8kA/src/PooledArrays.jl:451
...
```
The current implementation of `copy` directly uses the encoding data as the `pool`, but that `pool` does not contain `missing`:
```julia
julia> tb.A.encoding.data
2-element Arrow.List{Union{Missing, String}, Int32, Vector{UInt8}}:
"a"
"b"
```
This seems to be related to ongoing work on changing how the `missing` values should be represented in a `PooledArray`.

Start with the copy implementation for Arrow.DictEncoded and inspect how encoding.data is passed into the PooledArray pool. Reproduce the provided missing-value example, then verify that the pool represents missing and that accessing the copied column's missing element no longer raises UndefRefError.

Written by the indexing model from the issue text.