Avoid cloning empty arrays in java.lang.reflect.{Method,Constructor}

In `Method` and `Constructor`, we have two methods that clone the relevant arrays before returning them to external code:

    /**
     * {@inheritDoc}
     */
    @Override
    public Class<?>[] getParameterTypes() {
        return parameterTypes.clone();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Class<?>[] getExceptionTypes() {
        return exceptionTypes.clone();
    }

Frequently, those are empty arrays, and we do not really need to clone them. Note that JDK-8177107 did a related thing in shared parts: it would effectively populate these fields with a singleton empty class array.