feat(pgproto): support empty queries

src/pgproto/backend.rs

src/pgproto/backend/describe.rs

src/pgproto/backend/pgproc.rs

src/pgproto/backend/result.rs

src/pgproto/backend/storage.rs

src/pgproto/client/extended_query.rs

src/pgproto/client/simple_query.rs

src/pgproto/messages.rs

test/pgproto/extended_query_test.py

test/pgproto/simple_query_test.py