Allow inserting nothing as null value

[bryaan]

Instead of having to write conversion code for nothing => missing then wrap a bunch of calls in it can this lib just support inserting values of nothing as null?

[wookay]

about missing, NULL from docs
https://docs.julialang.org/en/v1/manual/missing/
"missing is equivalent to NULL in SQL and NA in R, and behaves like them in most situations."

there was a question,
How to pass a null value to a prepared statement with LibPQ.jl
https://discourse.julialang.org/t/how-to-pass-a-null-value-to-a-prepared-statement-with-libpq-jl/25068

testing NULL in test/adapters/postgresql/null.jl
https://github.com/wookay/Octo.jl/blob/master/test/adapters/postgresql/null.jl

but you could try this code that nothing for NULL

julia> using Octo.Adapters.PostgreSQL

julia> using Octo.AdapterBase: ElementStyle, SqlPartElement

julia> Octo.AdapterBase.sqlrepr(::Octo.DBMS.PostgreSQL, ::Nothing) = SqlPartElement(ElementStyle(:cyan), "NULL")

julia> to_sql([nothing])
"NULL"