Revert "Jenkins.getInstance() should be non-null"

This reverts commit bb7c8fcedb.

Closes #2090, I'll redo this as a PR... though if that PR is subject to multiple rounds of review before being merged then I will take that of evidence of the exact problem that committing directly was supposed to resolve... namely exponentially multiplying the effort required to make actual improvements to the code base.

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.getInstanceOrNull();
+        Jenkins j = Jenkins.getInstance();
         if (j != null) {
             for (Computer c : j.getComputers()) {
                 if (getChannel()==c.getChannel()) {