Regression since the Spoon 11 upgrade: any record crashes the verifier #369

Since #363 (Spoon 10.4.2 -> 11.5.0), any record in the checked code crashes verification, even without refinements:

public record Point(int x, int y) {
    int sum() { return x + y; }
}

On main 9045ecd:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.io.File.getAbsolutePath()" because the return value of "spoon.reflect.cu.SourcePosition.getFile()" is null
	at liquidjava.processor.refinement_checker.TypeChecker.scan(TypeChecker.java:74)
	at spoon.reflect.visitor.CtScanner.visitCtRecord(CtScanner.java:1065)

On be47cb6 (before the upgrade) it passes.

Cause: Spoon 11 dispatches records to visitCtRecord, not visitCtClass. ExternalRefinementTypeChecker skips classes (visitCtClass is empty) but not records, so it visits the record's methods with no external target (prefix is null): TypeFactory.createReference(null) throws. Reporting that error then fails too, because the record's implicit members have no source file, so TypeChecker.scan throws the NPE above instead of the real error.

Found re-checking our bank of real-bug examples on the new main (one of them is a record).