IGV: accept graphs without node categories

After JDK-8259984, IGV requires that graphs contain node category information, and fails with an "AssertionError: Node category not found, please use input from a compatible compiler version" otherwise (see attached screenshot). To support using the latest IGV on graphs generated from JDK 16 and earlier, the error could be replaced by a warning informing the user that some functionality (such as CFG approximation and some of the default filters) would not be available.