python: fix exception caused by default msg value
    
    This is a small fixup after PR#2024. If an exception is raised simply
    with `copr.v3.CoprRequestException()` we get another exception when
    trying to get its string representation:
    
        self = CoprRequestException(None)
    
            def __str__(self):
        >       errors = self.result.error.split("\n")
        E       AttributeError: 'NoneType' object has no attribute 'split'
    
    Merges: #2030