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/ProxyConfiguration.java

@@ -215,7 +215,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
      * This method should be used wherever {@link URL#openConnection()} to internet URLs is invoked directly.
      */
     public static URLConnection open(URL url) throws IOException {
-        Jenkins h = Jenkins.getInstance(); // this code might run on slaves
+        Jenkins h = Jenkins.getInstanceOrNull(); // this code might run on slaves
         ProxyConfiguration p = h!=null ? h.proxy : null;
         if(p==null)
             return url.openConnection();
@@ -241,7 +241,7 @@ public final class ProxyConfiguration extends AbstractDescribableImpl<ProxyConfi
     }
     
     public static InputStream getInputStream(URL url) throws IOException {
-        Jenkins h = Jenkins.getInstance(); // this code might run on slaves
+        Jenkins h = Jenkins.getInstanceOrNull(); // this code might run on slaves
         final ProxyConfiguration p = (h != null) ? h.proxy : null;
         if (p == null) 
             return new RetryableHttpStream(url);