Do not throw exceptions when wrapping an entity with a stale reference

When loading stale entity references via the metadata wrapper previously an exception was thrown. This is unfortunate when dealing with stale entity references as often contained in fields, though. Thus the behavior has been changed to not throw an exception but return NULL.

In detail, it works like that:

+   * Note that a list of entities might contain stale entity references. In
+   * that case the wrapper and the identifier of a stale reference would be
+   * still accessible, however the entity object value would be NULL. That way,
+   * there may be NULL values in lists of entity objects due to stale entity
+   * references.