Skip JDK8 TL Nightly build if there is no code change in JDK8 TL repo

JDK8 is "end-game" phase, only very small number of bug fixes will be integrated into JDK8, after b124, the jdk8 promotion build will be in "on demand" mode: JDK8 promotion build/test only happens when their are bug fixes integrated into JDK8.
To save engineering and machine resource, we also expect the JDK8 TL Nightly build only happens when bug fixes are integrated into JDK8 TL. If their is no bug fix integrated into JDK8 TL repo., the Nightly build will be skipped. Then, all tests will be skipped if there is no code change in JDK8 TL repo.