ignore empty equivalent terms (#4466)

src/collective/dms/thesaurus/browser/thesaurusview.py

@@ -105,6 +105,8 @@ class ListKeywordsView(BrowserView):
             self._items.append((normalized, obj.title, obj.id))
             titles.append(normalized)
             for equiv in (obj.equivs or []):
+                if not equiv:
+                    continue
                 normalized = normalizer.normalize(equiv).lower()
                 if normalized in  titles:
                     continue