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/FilePath.java

@@ -1133,7 +1133,7 @@ public final class FilePath implements Serializable {
      * @since 1.571
      */
     public @CheckForNull Computer toComputer() {
-        Jenkins j = Jenkins.getInstance();
+        Jenkins j = Jenkins.getInstanceOrNull();
         if (j != null) {
             for (Computer c : j.getComputers()) {
                 if (getChannel()==c.getChannel()) {