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>

.github/workflows/python.yml

@@ -61,6 +61,8 @@ jobs:
       run: |
         pip install -e .
         pip install tantivy@git+https://github.com/quickwit-oss/tantivy-py#164adc87e1a033117001cf70e38c82a53014d985
-        pip install pytest pytest-mock
+        pip install pytest pytest-mock black
+    - name: Black
+      run: black --check --diff --no-color --quiet .
     - name: Run tests
-      run: pytest -x -v --durations=30 tests
+      run: pytest -x -v --durations=30 tests