Jenkins.getInstance() will be @Nonnull in Jenkins 2.0+

- Code that is running from a plugin and on the master's JVM is guaranteed to never get null from this method (any cases where you do get null are bugs in core)
- Code that is running from a plugin and on a remote JVM should never be allowed to load the Jenkins class in their classloader, so should never use Jenkins.getInstance()... we are annotating the method with @Nullable so that such code can have some evolution time
- Code that is running in core and on one of two special paths should use the Jenkins.getInstanceOrNull() method so that the UI can be presented to users before the singleton has been instantiated / after the singleton has been destroyed
- The remaining 95% of uses in core (and 100% of uses in plugins) can safely assume that the instance is never null