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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _get_request_body_bytes_only(param_name, param_value):
    '''Validates the request body passed in and converts it to bytes
    if our policy allows it.'''
    if param_value is None:
        return b''

    if isinstance(param_value, bytes):
        return param_value

    raise TypeError(_ERROR_VALUE_SHOULD_BE_BYTES.format(param_name))