VersionUtil.versionFor() unexpectedly return null instead of Version.unknownVersion() stated in JavaDoc

[sammyhk]

As titled, after commit of 88c296c , it unexpectedly return null instead of Version.unknownVersion() which cause our application broken after upgrade from 2.4.x to 2.6.x.

[cowtowncoder]

Thank you for reporting this. Looks like unintended consequence of refactoring; will fix.