RFR: 8159031: jjs throws NoSuchFileException if ~/.jjs.history does not exist

[Hannes Wallnöfer]

Please review the following changes:

Bug: https://bugs.openjdk.java.net/browse/JDK-8159031
JDK webrev: http://cr.openjdk.java.net/~hannesw/8159031/jdk/webrev.00/
Nashorn webrev: http://cr.openjdk.java.net/~hannesw/8159031/nashorn/webrev.00/

These are two one-liners that add a file existence check in Nashorn and a null check in jline.

Thanks, 
Hannes