Fix Tables schema for empty layers

[evetion]

Fixes #440

This lets Tables.schema return an actual schema, but only when no features are present.

If we always use the schema, we would need to also determine whether features attributes (columns) can be missing, and set the types accordingly. Without scanning all data (which is bad), this would result in all isnullable fields becoming Union{Missing,T} (also bad).

[yeesian]

Thanks for this! Can we also add a test for it in https://github.com/yeesian/ArchGDAL.jl/blob/master/test/test_tables.jl ?

[evetion]

Thanks for this! Can we also add a test for it in master/test/test_tables.jl ?

Done! And I fixed some macros, as interpolating did not work correctly (most @ogrerr statements don't interpolate their string message, but deletefeature! does.

[yeesian]

It looks great, thank you!