Regression in getImplementationVersion from Jigsaw

[Michael D]

Hi All,

Hope I'm in the right place.
The implementation of Jigsaw caused a regression in that
Package.getImplementationVersion is unusable if no other version
things are defined in the manifest.
This appears to be an unintentional regression caused by a missing null check.
I have prepared a patch and unit test for review here:
https://patch-diff.githubusercontent.com/raw/md-5/OpenJDK/pull/3.patch
Note that only testImplVersion fails, but I have included tests for
all the version elements for completeness.

NB: Not an OpenJDK dev so can't create webrev or bug report.

Thanks
Michael