PaginatedQuery: set a default for _sort_column

Previously, _sort_column wasn't set by default and needed to be set by
the query, but some of the other methods would fail if it hadn't been
set. This just defaults it to use the created_time column as the default
sort, which should always be present on models being queried by this
class (for now, at least).

tildes/tildes/models/pagination.py

@@ -22,7 +22,8 @@ class PaginatedQuery(ModelQuery):
 
         super().__init__(model_cls, request)
 
-        self._sort_column: Optional[Column] = None
+        # default to sorting by created_time descending (newest first)
+        self._sort_column = model_cls.created_time
         self.sort_desc = True
 
         self.after_id: Optional[int] = None
@@ -135,7 +136,6 @@ class PaginatedQuery(ModelQuery):
         """Finalize the query before execution."""
         query = super()._finalize()
 
-        if self._sort_column:
         # if the query is reversed, we need to sort in the opposite dir
         # (basically self.sort_desc XOR self.is_reversed)
         desc = self.sort_desc