[JENKINS-13202] 646154f refinement: resolveSymlink is documented to return null, not throw an exception, when called on a regular file.

core/src/test/java/hudson/UtilTest.java

@@ -171,6 +171,7 @@ public class UtilTest {
         File d = Util.createTempDir();
         try {
             new FilePath(new File(d, "a")).touch(0);
+            assertNull(Util.resolveSymlink(new File(d, "a")));
             Util.createSymlink(d,"a","x", l);
             assertEquals("a",Util.resolveSymlink(new File(d,"x")));