compile_files return error when calling with --output-dir=<path> param

[jualy007]

• py-solc Version: 3.2.0
• solc Version: 0.4.21
• Python Version: 3.6.5
• OS: linux

When calling compile_files method with --output-dir= param:

stdoutdata, stderrdata, command, proc = solc_wrapper(**compiler_kwargs)

stdoutdata is None, then contracts = _parse_compiler_output(stdoutdata) will throw exception in code:
def _parse_compiler_output(stdoutdata):
output = json.loads(stdoutdata)