RFR:JDK-8155581:jshell tool: replace use of Option.get()

[nadeesh tv]

Hi all,

Please review

BugId :  https://bugs.openjdk.java.net/browse/JDK-8155581

webrev:  http://cr.openjdk.java.net/~ntv/kulla/8155581/webrev.00/

Solution : Changed the usage of stream to String.join() as suggested by 
Stuart

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Thanks and Regards,
Nadeesh TV