[BACKLOG-175]: A corrupt repository object leaves the repository explorer in an unusable state.

There is an issue in JCR: NPE when calling node.getBaseVersion()(please see https://issues.apache.org/jira/browse/JCR-2382)
So the fix consists of catching NPE from JRC, logging the error and returnig null value for Version.
Also added several unit tests.
Merge pull request #1724 from TatsianaKasiankova/BACKLOG-175

[BACKLOG-175]: A corrupt repository object leaves the repository explorer in an unusable state.