Do not add --jar if JAR_PATH is empty

[alubbe]

This PR has come out of the suggestion to investigate nailgun google/closure-compiler#2186

fast-closure-compiler passes the arguments to a python script which in turn passes it to the nailgun server. -jar gets passed all the way down to the closure compiler, resulting in this error:

"-jar" is not a valid option

Therefore, I suggest to not add -jar if JAR_PATH is falsy. Library consumers like me then have a switch to extend this libraries' functionality to work with nailgun.

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[alubbe]

I signed it!

[googlebot]

CLAs look good, thanks!

[ChadKillingsworth]

Works for me.