Fix issue #702: Handle CSV with only header row in --detect-types (#707)

When inserting a CSV file that contains only a header row (no data rows),
the --detect-types option would crash with an AssertionError because it
tried to transform a table that didn't exist.

This fix adds a check to ensure the table exists before attempting to
apply type transformations. The fix is applied in two places:
1. insert_upsert_implementation() for the insert command
2. memory() command for CSV/TSV files

Added test case: test_insert_csv_headers_only

Co-authored-by: Test User <test@example.com>

tests/test_cli_insert.py

@@ -597,3 +597,21 @@ def test_insert_streaming_batch_size_1(db_path):
     proc.stdin.close()
     proc.wait()
     assert proc.returncode == 0
+
+
+def test_insert_csv_headers_only(tmpdir):
+    """Test that CSV with only header row (no data) works with --detect-types (issue #702)"""
+    db_path = str(tmpdir / "test.db")
+    csv_path = str(tmpdir / "headers_only.csv")
+    with open(csv_path, "w") as fp:
+        fp.write("id,name,age\n")
+    # Should not crash with --detect-types (which is now the default)
+    result = CliRunner().invoke(
+        cli.cli,
+        ["insert", db_path, "data", csv_path, "--csv"],
+        catch_exceptions=False,
+    )
+    assert result.exit_code == 0
+    # Table should not exist since there were no data rows
+    db = Database(db_path)
+    assert not db["data"].exists()