After deleting a page XWikiDocument.java's getObjectNumbers() returns the wrong value if the page is still cached

If a page containing objects is deleted, the method getObjectNumbers(String classname) may return a wrong value (instead of 0) as the vector used may null elements after the delete of the page (if still in the cache) which are counted in the returned value.

Test case:

1. Create a page containing objects
2. Delete the page
3. Examine the values of the following (replacing PAGE_NAME and OBJECT_CLASS as appropriate)
$xwiki.getDocument("PAGE_NAME").getObjectNumbers("OBJECT_CLASS")
$xwiki.getDocument("PAGE_NAME").getObjects("OBJECT_CLASS")