private ValidationEvent emit(
            Severity severity,
            String eventIdSuffix,
            ShapeId shape,
            SourceLocation sourceLocation,
            String prefixMessage,
            String message
    ) {
        String actualId = eventIdSuffix == null ? getEventId() : (getEventId() + '.' + eventIdSuffix);
        String actualMessage = prefixMessage == null ? message : (prefixMessage + "; " + message);
        return ValidationEvent.builder()
                .id(actualId)
                .shapeId(shape)
                .sourceLocation(sourceLocation)
                .message(actualMessage)
                .severity(severity)
                .build();
    }
Validates that only backward compatible changes are made to
structure member nullability to ensure that if something was
previously nullable to clients then it continue to be nullable
and vice versa.
smithy-diff/src/main/java/software/amazon/smithy/diff/evaluators/ChangedNullability.java