True if only a single empty `a:p` element is present.

def is_empty(self):
        """True if only a single empty `a:p` element is present."""
        ps = self.p_lst
        if len(ps) > 1:
            return False

        if not ps:
            raise InvalidXmlError('p:txBody must have at least one a:p')

        if ps[0].text != '':
            return False
        return True