RFR: 8068790: ZipEntry/JarEntry.setCreation/LastAccessTime(null) don't throw NPE as specified

[Claes Redestad]

Hi all,

please review this simple fix to do the right null checks in a few 
ZipEntry methods.

bug: https://bugs.openjdk.java.net/browse/JDK-8068790
webrev: http://cr.openjdk.java.net/~redestad/jdk9/8068790/webrev.0/

/Claes