Performs invalid character removal and whitespace cleanup on text.

def _clean_text(self, text):
        """Performs invalid character removal and whitespace cleanup on text."""
        output = []
        for char in text:
            cp = ord(char)
            if cp in (0, 0xfffd) or self._is_control(char):
                continue
            if self._is_whitespace(char):
                output.append(' ')
            else:
                output.append(char)
        return ''.join(output)