Fix state sequence algorithm edge case

Credits to @vanh00f

Author: GJFR

bughog/search_strategy/bgb_search.py

@@ -86,6 +86,8 @@ def __get_next_pair_to_split(self) -> Optional[tuple[State, State]]:
         # Remove the first and last pair if they have a first and last state without a result, respectively
         if pairs[0][0].result_variables is None:
             pairs = pairs[1:]
+        if not pairs:
+            return None
         if pairs[-1][1].result_variables is None:
             pairs = pairs[:-1]
         # Remove all pairs that have already been identified as unavailability gaps