Fix for empty arrays

[tlienart]

scitype_union is now

function scitype_union(A)
    isempty(A) && return scitype(eltype(A))
    reduce((a,b)->Union{a,b}, (scitype(el) for el in A))
end

before the first line wasn't there and so potentially there was a reduction over an empty array.

closes #107

[codecov-commenter]

Codecov Report

Merging #108 into master will increase coverage by 0.08%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   96.96%   97.05%   +0.08%     
==========================================
  Files           2        2              
  Lines          33       34       +1     
==========================================
+ Hits           32       33       +1     
  Misses          1        1              

Impacted Files	Coverage Δ
src/scitype.jl	100.00% <100.00%> (ø)

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[DilumAluthge]

Maybe also add a test for Missing[]?

[DilumAluthge]

Thanks for such a quick fix!