test(python): cover schema-only vector table creation (#3882)

## Summary

- make the existing #1968 regression explicitly assert that schema-only
table creation succeeds
- verify the new table has zero rows and preserves the requested
fixed-size vector schema before accepting subsequent data

## Root cause

In v0.16.0, schema-only table creation sent an empty table through
vector sanitization, which calculated a remainder using `len(data)` and
raised `ZeroDivisionError`. Later refactors removed that runtime path,
but the issue-specific regression only asserted the final row count
after a subsequent add. This change makes the reported operation and its
expected empty-table state explicit so the original defect remains
directly covered.

## Validation

- `uv run --extra tests pytest
python/tests/test_table.py::test_create_table_without_data_with_vector_schema
-q`
- `uv --project python run --extra tests --extra dev ruff format --check
python/python/tests/test_table.py`
- `uv --project python run --extra tests --extra dev ruff check .`
- `git diff --check`

Fixes #1968

<!-- lance-gatekeeper-fix:v1 agent=b8ec6f40f4bba2f9beeaaae12233e5c4
generation=1 -->

Co-authored-by: Gatefixer <313497061+lancedb-gatefixer[bot]@users.noreply.github.com>

python/python/tests/test_table.py

@@ -3489,8 +3489,8 @@ def test_create_table_empty_list_no_schema_error(mem_db: DBConnection):
         mem_db.create_table("test_empty_no_schema", data=[])
 
 
-def test_add_table_with_empty_embeddings(tmp_path):
-    """Test exact scenario from issue #1968
+def test_create_table_without_data_with_vector_schema(tmp_path):
+    """Test exact scenario from issue #1968.
 
     Regression test for issue #1968:
     https://github.com/lancedb/lancedb/issues/1968
@@ -3502,6 +3502,9 @@ def test_add_table_with_empty_embeddings(tmp_path):
         embedding: Vector(16)
 
     table = db.create_table("test", schema=MySchema)
+    assert table.count_rows() == 0
+    assert table.schema == MySchema.to_arrow_schema()
+
     table.add(
         [{"text": "bar", "embedding": [0.1] * 16}],
         on_bad_vectors="drop",