Fix an overly aggressive assertion.

This could happen when attempting to evaluate `IS_CHANGED` for a node
during the creation of the cache (in order to create the cache key).

comfy/caching.py

@@ -172,7 +172,8 @@ def _set_immediate(self, node_id, value):
         self.cache[cache_key] = value
 
     def _get_immediate(self, node_id):
-        assert self.cache_key_set is not None
+        if self.cache_key_set is None:
+            return None
         cache_key = self.cache_key_set.get_data_key(node_id)
         if cache_key in self.cache:
             return self.cache[cache_key]