An Objective-C category whose class field is null, or is bound to a
symbol other than `_OBJC_CLASS_$_`, was recorded as partial with
`category_class_unresolved`, but no issue was raised, so the
`objc_properties_categories` coverage facet reported `partial` with a
null reason. swiftc emits such categories for @objc members of an
extension on another module's Swift class and binds the class field to
the class metadata symbol (`_$s…CN`); Keynote and Notability carry them.
Report the reference as a `category_class_unresolved` issue naming the
bound symbol, or stating that the reference is null, matching the issue
already raised for an unreadable local class reference.
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