Support the new non nil strings in nim 0.19.

[treeform]

Getting this error now:

asyncpg/apg_core.nim(528, 9) template/generic instantiation from here
apg_core.nim(503, 14) Error: type mismatch: got <nil> but expected 'string'

[treeform]

see #8

[treeform]

I ended up writing my own async pg lib that does not have this issue here: https://github.com/treeform/pg