Ensure that CompondValue class as a __values__ attribute in __init__

[dagostinelli]

Looking in valueobjects.py, around here: https://github.com/mvantellingen/python-zeep/blob/c80519ef01216f8e5bcfe8de7995d841e03b0f2a/src/zeep/xsd/valueobjects.py#L91

I see:

class CompoundValue:
    """Represents a data object for a specific xsd:complexType."""

    _xsd_type: "ComplexType"
    _xsd_elm: "Element"

    def __init__(self, *args, **kwargs):
        values = OrderedDict()

        # Can be done after unpickle
        if self._xsd_type is None:
            return

        # <omit>
        self.__values__ = values        

Since there's an early return, the self.__values__ attribute does not exist.

Looking at the rest of the class, I see that many of the dunder methods are looking for self.__values__ attribute (ex: __repr__ looks for it)

It could be the cause for #1155 and a similar error that I am seeing. In my case, my code is hitting __repr__ and the object doesn't have __values__ attribute yet.

I think the fix is to ensure that __values__ attribute exists before returning from __init__