Jenkins.getInstance() should be non-null

- It is never too late to do the right thing.
- The vast majority of usages of `Jenkins.getInstance()` in core currently assume that its return value is non-null
- This commit changes those that are written to correctly check for non-null values will call `Jenkins.getInstanceOrNull()`
- We deprecate the `Jenkins.getActiveInstance()` madness
- I checked with @kohsuke who said not to bother with a PR and just commit this strongly opinionated change direct to master as a PR will just degrade into a bikeshedding.

core/src/main/java/hudson/WebAppMain.java

@@ -236,7 +236,7 @@ public class WebAppMain implements ServletContextListener {
                     } catch (Exception e) {
                         new HudsonFailedToLoad(e).publish(context,_home);
                     } finally {
-                        Jenkins instance = Jenkins.getInstance();
+                        Jenkins instance = Jenkins.getInstanceOrNull();
                         if(!success && instance!=null)
                             instance.cleanUp();
                     }
@@ -392,7 +392,7 @@ public class WebAppMain implements ServletContextListener {
 
     public void contextDestroyed(ServletContextEvent event) {
         terminated = true;
-        Jenkins instance = Jenkins.getInstance();
+        Jenkins instance = Jenkins.getInstanceOrNull();
         if(instance!=null)
             instance.cleanUp();
         Thread t = initThread;