missing: restrict length(::SkipMissing) to only AbstractArray

`length(::SkipMissing)`, as defined, can actually change the length
of some objects (like stateful iterators [1]).

Thus, restrict the method to work only on `AbstractArray`.

[1] `length(::SkipMissing)` changing the length of a `Stateful` iterator.
```
julia> itr = Iterators.Stateful([missing, 1, missing])
Base.Iterators.Stateful{Vector{Union{Missing, Int64}}, Union{Nothing, Tuple{Union{Missing, Int64}, Int64}}}(Union{Missing, Int64}[missing, 1, missing], (missing, 2), 0)

julia> count(!ismissing, skipmissing(itr).x) # proposed definition of length
1

julia> count(!ismissing, skipmissing(itr).x)
0
```

Author: thazhemadam

base/missing.jl

@@ -246,7 +246,7 @@ IteratorSize(::Type{<:SkipMissing}) = SizeUnknown()
 IteratorEltype(::Type{SkipMissing{T}}) where {T} = IteratorEltype(T)
 eltype(::Type{SkipMissing{T}}) where {T} = nonmissingtype(eltype(T))
 
-length(s::SkipMissing) = count(!ismissing, s.x)
+length(s::SkipMissing{T}) where T <: AbstractArray = count(!ismissing, s.x)
 
 function iterate(itr::SkipMissing, state...)
     y = iterate(itr.x, state...)