[formatter] use value of last alternative (#4492)

fixes {fieldname|''} evaluating to the value of 'keywords-default'
instead of an empty string

gallery_dl/formatter.py

@@ -182,9 +182,10 @@ class StringFormatter():
                     if obj:
                         break
                 except Exception:
-                    pass
+                    obj = None
             else:
-                obj = self.default
+                if obj is None:
+                    obj = self.default
             return fmt(obj)
         return wrap