fetch_next and fetch_previous shouldn't crash on last page

When `fetch_next` reaches the last page, the `_pagination_next`
attribute exists in the page structure but is set to None; similarly
for `_pagination_prev` in `fetch_previous`. If this occurs we need to
return `None` rather than trying to interate over `None` and
generating an exception.

mastodon/utility.py

@@ -134,6 +134,9 @@ class Mastodon(Internals):
         else:
             params = copy.deepcopy(previous_page)
 
+        if params is None:
+            return None
+
         is_pagination_dict = False
         if isinstance(previous_page, dict):
             if all(key in ['_pagination_method', '_pagination_endpoint', 'min_id', 'max_id', 'since_id', 'limit'] for key in previous_page):
@@ -173,6 +176,9 @@ class Mastodon(Internals):
         else:
             params = copy.deepcopy(next_page)
 
+        if params is None:
+            return None
+
         is_pagination_dict = False
         if isinstance(next_page, dict):
             if all(key in ['_pagination_method', '_pagination_endpoint', 'min_id', 'max_id', 'since_id', 'limit'] for key in next_page):