Creation of missing entry even if it is not present

[bkamins]

I think in such cases missing should not appear as level in the output:

Case 1:

julia> freqtable(categorical(["a", 'b']))
3-element Named Array{Int64,1}
Dim1    │
────────┼──
a       │ 1
'b'     │ 1
missing │ 0

Case 2:

julia> freqtable(categorical(Union{String,Missing}["a", "b"]))
3-element Named Array{Int64,1}
Dim1    │
────────┼──
a       │ 1
b       │ 1
missing │ 0

[nalimilan]

Good catch. Shouldn't be too hard to fix.