fix: don't use a list for empty arguments

[IlyaFaer]

On SQLAlchemy dialect label I've noticed an error:

File "/home/runner/work/python-spanner-sqlalchemy/python-spanner-sqlalchemy/.nox/compliance_test_13/lib/python3.8/site-packages/google/cloud/spanner_dbapi/connection.py", line 437, in run_statement
    _execute_insert_heterogenous(
  File "/home/runner/work/python-spanner-sqlalchemy/python-spanner-sqlalchemy/.nox/compliance_test_13/lib/python3.8/site-packages/google/cloud/spanner_dbapi/_helpers.py", line 53, in _execute_insert_heterogenous
    transaction.execute_update(sql, params, get_param_types(params))
  File "/home/runner/work/python-spanner-sqlalchemy/python-spanner-sqlalchemy/.nox/compliance_test_13/lib/python3.8/site-packages/google/cloud/spanner_dbapi/parse_utils.py", line 287, in get_param_types
    for key, value in params.items():
AttributeError: 'list' object has no attribute 'items'

This tiny change fixes the error.