8297682: Use Collections.emptyIterator where applicable

Reviewed-by: stsypanov, alanb, jpai

src/java.base/unix/classes/sun/nio/fs/UnixFileSystem.java

@@ -189,8 +189,7 @@ abstract class UnixFileSystem
                         sm.checkRead(rootDirectory.toString());
                     return allowedList.iterator();
                 } catch (SecurityException x) {
-                    List<Path> disallowed = Collections.emptyList();
-                    return disallowed.iterator();
+                    return Collections.emptyIterator(); //disallowed
                 }
             }
         };