[Go] Fix nullable models embedded in allOf

Created by: impl

It is possible, though contrived, for a property to have both isAnyType and isModel set. In this case, when the outer type is nullable, the Go template expects a Nullable* type to be provided by the generator, but it is skipped over instead. This change aligns the generator with the template's expectations.

Note that without these changes, the generated Go code will not compile. I've included a test to demonstrate the expected behavior.

Also, for debugging purposes, it was helpful to have the value of isAnyType printed when dumping a CodegenProperty. In fact, it was missing from equals and hashCode too, so I updated it. That change doesn't have any impact on the generators though as far as I can tell and I'm happy to back that bit out if needed.

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