ERP5Type.Base: Tolerate _baseGetTitle absence.

Although it should never happen in normal use, it may happen during an
upgrade when property sheets fail creating accessors (ex: broken object
because portal type moved to a different Business Template).

product/ERP5Type/Base.py

@@ -2366,9 +2366,7 @@ class Base( CopyContainer,
       r = self.getShortTitle()
       if r: return r
     return (self.getProperty('reference') or
-            # No need to test existence since all Base instances have this method
-            # Also useful whenever title is calculated
-            self._baseGetTitle() or
+            getattr(self, '_baseGetTitle', str)() or
             self.getId())
 
   security.declareProtected(Permissions.AccessContentsInformation,