In some case, currentInterfaceName can be null. Take that into account

Without this PR, checkiid is failing with 
``̀`̀
WARNING: 'nsIDOMMobileMessageManager.idl' was not found in local repository. Are you sure your repository is at the correct revision?
Traceback (most recent call last):
  File "../checkiid/checkiid.py", line 934, in <module>
    runMain()
  File "../checkiid/checkiid.py", line 928, in runMain
    main(rootPath, patchFile)
  File "../checkiid/checkiid.py", line 854, in main
    interfaceFilename = interfaceNameIDLMap[interface]
KeyError: None
``̀`̀
on this patch:
```
diff --git a/dom/mobilemessage/interfaces/nsIDOMMobileMessageManager.idl b/dom/mobilemessage/interfaces/nsIDOMMobileMessageManager.idl
--- a/dom/mobilemessage/interfaces/nsIDOMMobileMessageManager.idl
+++ b/dom/mobilemessage/interfaces/nsIDOMMobileMessageManager.idl
@@ -73,9 +73,10 @@ interface nsIDOMMozMobileMessageManager 
   [implicit_jscontext] attribute jsval onretrieving;
   [implicit_jscontext] attribute jsval onsending;
   [implicit_jscontext] attribute jsval onsent;
   [implicit_jscontext] attribute jsval onfailed;
   [implicit_jscontext] attribute jsval ondeliverysuccess;
   [implicit_jscontext] attribute jsval ondeliveryerror;
   [implicit_jscontext] attribute jsval onreadsuccess;
   [implicit_jscontext] attribute jsval onreaderror;
+  [implicit_jscontext] attribute jsval ondeleted;
 };
```