Allow creation of an empty table (#254)

It's inconvenient to always require data at table creation time.
Here we enable you to create an empty table and add data and set schema
later.

---------

Co-authored-by: Chang She <chang@lancedb.com>

python/lancedb/conftest.py

@@ -1,10 +1,8 @@
-import builtins
 import os
 
 import pytest
 
 # import lancedb so we don't have to in every example
-import lancedb
 
 
 @pytest.fixture(autouse=True)