Allow `ATTACH TABLE [db.]name` if the table was previously detached,
an the table structure can be read from disk. This makes reattaching
tables less cumbersome:
```
CREATE TABLE test.t (x UInt8) ENGINE = Null;
DETACH TABLE test.t;
ATTACH TABLE test.t;
```
This allows creation of materialized views without inner tables,
using an existing table for materialized rows instead.
This is useful for cases when you want to detach the materializing
view, but keep the already materialized data readable, especially
when the inner table is replicated.
