Describe the Bug
If the search_path is empty, CURRENT_SCHEMA() returns NULL but the reader tries to read a string so it fails with:

*database.Error: sql: Scan error on column index 0, name "current_schema": converting NULL to string is unsupported in line 0: SELECT CURRENT_SCHEMA()

Steps to Reproduce
Steps to reproduce the behavior:

1. Give it a DSN with empty search path
2. Run migrate
3. It will fail badly

Expected Behavior
Fail better returning ErrNoSchema, in

Migrate Version
v4.15.0

Loaded Database Drivers
postgres

Go Version
go version go1.17.2 darwin/arm64

Additional context
Should just scan into *string allowing nulls to work.

cq=# set search_path='';
SET
cq=# SELECT CURRENT_SCHEMA();
 current_schema
----------------

(1 row)

cq=# SELECT CURRENT_SCHEMA() is null;
 ?column?
----------
 t
(1 row)