perf: don't eval attr just pass to isinstance

[TonisPiip]

Had a nasty issue where I had a large QS be a @class_property, and this code caused it to eval the QS, there's no need to eval it, just pass it directly do isinstance, if we really want a check, a is not None would be better.

But would be even better if this didn't loop over all properities, rather it used

cls._meta.related_objects

[TonisPiip]

cls._meta._get_fields(reverse=True, forward=False) would be a better option,

I've swtiched to using class_method to avoid this issue, but this was a nasty surprise. :( a warning about using class_property in the docs would be good.