[9-test-only] RFR of 8135091: (fs) java/nio/file/Files/StreamLinesTest.java should test empty files

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8135091
Webrev:	http://cr.openjdk.java.net/~bpb/8135091/webrev.00/

Summary:
In lines() add zero length case and rearrange first loop to avoid duplicate cases.

Thanks,

Brian