<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _put_output(self, output: Optional[str]) -> None: """Put output string to ffmpeg command."""

        if output is None:
            self._argv.extend(["-f", "null", "-"])
            return

        output_cmd = shlex.split(str(output))
        if len(output_cmd) > 1:
            self._argv.extend(output_cmd)
        else:
            self._argv.append(output)