Fix copyto! with #undef

[bkamins]

This fixes the following problem:

julia> x = PooledArray(["a", "b"])
2-element PooledVector{String, UInt32, Vector{UInt32}}:
 "a"
 "b"

julia> y = resize!(PooledArray(String[]), 2)
2-element PooledVector{String, UInt32, Vector{UInt32}}:
 #undef
 #undef

julia> copyto!(x, y)
ERROR: UndefRefError: access to undefined reference

Then maybe we also need to add in this PR

Base.copyto!(dest::PooledArrOrSub{T, N, R}, doffs::Union{Signed, Unsigned},
                      src::AbstractArray, soffs::Union{Signed, Unsigned},
                      n::Union{Signed, Unsigned}) where {T, N, R}

method depending on how JuliaLang/julia#45125 is resolved.

CC @nalimilan

[codecov]

Codecov Report

Merging #83 (ad1f384) into main (65e3316) will decrease coverage by 0.23%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
- Coverage   89.22%   88.98%   -0.24%     
==========================================
  Files           1        1              
  Lines         334      336       +2     
==========================================
+ Hits          298      299       +1     
- Misses         36       37       +1     

Impacted Files	Coverage Δ
src/PooledArrays.jl	88.98% <66.66%> (-0.24%)	⬇️

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[nalimilan]

Good catch, but apparently we need to wait for the result of the discussion at JuliaLang/julia#45125 to know whether this behavior should be supported or not, right?

[bkamins]

Right.

My assumption is that in JuliaLang/julia#45125 the conclusion will be: propagate #undef if it is possible, and throw an error otherwise.