Add allocate(::Type{Null})

[alyst]

Otherwise allocate(::Type{T}, rows, ref) where {T <: Union{WeakRefString, Null}} method gets called resulting in an error.

[nalimilan]

Is there a way to test this?

[alyst]

Originally I've encountered it while trying to CSV.read() a table with all-nulls columns. It fails in DataFrame(schema) call that should create a DataFrameStream() object.
That's what I've added to the tests. But since this API is new to me, I just don't know whether it's a valid code pattern or the user code triggering this path should look different.

[coveralls]

Coverage increased (+0.3%) to 88.358% when pulling 7307a89 on alyst:allocate_nulls into 75ad2fe on JuliaData:master.

[coveralls]

Coverage increased (+0.3%) to 88.358% when pulling 7307a89 on alyst:allocate_nulls into 75ad2fe on JuliaData:master.

[quinnj]

This looks good. Thanks for the addition.