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 @@ public Iterator<Path> iterator() {
                         sm.checkRead(rootDirectory.toString());
                     return allowedList.iterator();
                 } catch (SecurityException x) {
-                    List<Path> disallowed = Collections.emptyList();
-                    return disallowed.iterator();
+                    return Collections.emptyIterator(); //disallowed
                 }
             }
         };

src/jdk.compiler/share/classes/com/sun/tools/javac/processing/JavacProcessingEnvironment.java

@@ -383,8 +383,7 @@ private Iterator<Processor> handleServiceLoaderUnavailability(String key, Except
             handleException(key, e);
         }
 
-        java.util.List<Processor> pl = Collections.emptyList();
-        return pl.iterator();
+        return Collections.emptyIterator();
     }
 
     /**