fixes unnest_wider bug w missing key

[drizk1]

address #142 found here

julia> a1=Dict("a"=>1, "b"=>Dict("c"=>1, "d"=>2)); a2=Dict("a"=>1, "b"=>Dict("c"=>1)); a=[a1;a2]; df=DataFrame(a);

julia> @unnest_wider(df, b)
2×3 DataFrame
 Row │ a      c      d       
     │ Int64  Int64  Int64?  
─────┼───────────────────────
   1 │     1      1        2
   2 │     1      1  missing 

[drizk1]

addresses an edge case in the discourse post

julia> a0=Dict("a"=>0, "b"=>0);  a1=Dict("a"=>1, "b"=>Dict("c"=>1, "d"=>2)); a2=Dict("a"=>2, "b"=>Dict("c"=>2)); a3=Dict("a"=>3, "b"=>Dict("c"=>3));

julia> a=[a0;a1;a2;a3];

julia> df3=DataFrame(a)
4×2 DataFrame
 Row │ b                     a     
     │ Any                   Int64 
─────┼─────────────────────────────
   1 │ 0                         0
   2 │ Dict("c"=>1, "d"=>2)      1
   3 │ Dict("c"=>2)              2
   4 │ Dict("c"=>3)              3

julia> @unnest_wider(df3, b)
4×3 DataFrame
 Row │ a      c        d       
     │ Int64  Int64?   Int64?  
─────┼─────────────────────────
   1 │     0  missing  missing 
   2 │     1        1        2
   3 │     2        2  missing 
   4 │     3        3  missing