Convert NULL type columns to Julia Missing type

[TacHawkes]

At the moment columns which are NULL type are returned as Julia type "Any" which is suboptimal for filtering missing values when reading values iteratively.
With this PR a NULL type column is handled as a Missing type in Julia.

[quinnj]

Thanks @TacHawkes!

[quinnj]

It looks like one of the tests is failing; would you mind taking a look? https://github.com/JuliaDatabases/SQLite.jl/blob/master/test/runtests.jl#L107

[TacHawkes]

It looks like one of the tests is failing; would you mind taking a look? https://github.com/JuliaDatabases/SQLite.jl/blob/master/test/runtests.jl#L107

Sure, the problem is the testcase itself in my opinion. Without the PR, the test "empty query has correct schema and return type:" finds the following types for all_tbl and empty_tbl and the corresponding type comparison:

empty_tbl type, all_tbl type
Union{Missing, Int64}, Int64 -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, Int64}, Union{Missing, Int64} -> true
Any, String -> true
Any, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true

With the pull request:

empty_tbl type, all_tbl type
Union{Missing, Int64}, Int64 -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, Int64}, Union{Missing, Int64} -> true
Missing, String -> false
Missing, String -> false
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true
Union{Missing, String}, String -> true

As Missing >: String is false. Simple solution (but valid!?):

@test all(col -> eltype(empty_tbl[col]) == Missing || eltype(empty_tbl[col]) >: eltype(all_tbl[col]), propertynames(all_tbl))

What do you think?

[quinnj]

Yeah, that sounds better to me!

[quinnj]

Put the fix in a PR w/ the test fix here: #261