insert: don't crash on header-only CSV with --detect-types

When the input has a header but no data rows, the insert path never
creates the table - the row loop has nothing to feed into the first
insert. The post-insert tracker.transform call then asserts the table
exists and blows up:

    AssertionError: Cannot transform a table that doesn't exist yet

Reported in #702 with the exact fix in the body. Guard the transform
on db[table].exists() so an empty CSV is just a no-op, matching what
--detect-types already does for a zero-byte CSV (no header either).

Closes #702.

Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>

sqlite_utils/cli.py

@@ -1175,7 +1175,10 @@ def insert_upsert_implementation(
                 )
             else:
                 raise
-        if tracker is not None:
+        # An empty CSV (header row but no data rows) never creates the table,
+        # so there's nothing to transform. Without this guard, --detect-types
+        # crashes on header-only input (see #702).
+        if tracker is not None and db[table].exists():
             db.table(table).transform(types=tracker.types)
 
         # Clean up open file-like objects

tests/test_cli.py

@@ -2318,6 +2318,23 @@ def test_upsert_detect_types(tmpdir, option):
     ]
 
 
+@pytest.mark.parametrize("option", ("-d", "--detect-types"))
+def test_insert_csv_header_only_with_detect_types(tmpdir, option):
+    """Header-only CSVs (no data rows) used to crash --detect-types because
+    the transform step ran against a table that was never created. Now we
+    skip transform when there's nothing to transform. (#702)"""
+    db_path = str(tmpdir / "test.db")
+    result = CliRunner().invoke(
+        cli.cli,
+        ["insert", db_path, "empty", "-", "--csv", option],
+        catch_exceptions=False,
+        input="name,age,weight\n",
+    )
+    assert result.exit_code == 0
+    db = Database(db_path)
+    assert "empty" not in db.table_names()
+
+
 def test_csv_detect_types_creates_real_columns(tmpdir):
     """Test that CSV import creates REAL columns for floats (default behavior)"""
     db_path = str(tmpdir / "test.db")