Prevent NullPointerException in ACL parser

Parsing `{}` as ACL resulted in a NullPointerException since Opencast
just expected the object to contain a field `acl`. That means, something
like `{"acl":{}}` would be fine and result in an empty ACL. The empty
object should be handled gracefully and be treated the same. That is,
what this patch does.

modules/common/src/main/java/org/opencastproject/security/api/AccessControlParser.java

@@ -136,11 +136,14 @@ public static AccessControlList parseAcl(InputStream in) throws IOException, Acc
    */
   private static AccessControlList parseJson(String content) throws AccessControlParsingException {
     try {
+      AccessControlList acl = new AccessControlList();
       JSONObject json = (JSONObject) new JSONParser().parse(content);
       JSONObject jsonAcl = (JSONObject) json.get(ACL);
+      if (jsonAcl == null) {
+        return acl;
+      }
       Object jsonAceObj = jsonAcl.get(ACE);
 
-      AccessControlList acl = new AccessControlList();
       if (jsonAceObj == null)
         return acl;