Fix wrong type hint

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/timeseries/_formula_engine/_formula_steps.py

@@ -145,7 +145,7 @@ def __repr__(self) -> str:
         """
         return "("
 
-    def apply(self, _: List[Optional[float]]) -> None:
+    def apply(self, _: List[float]) -> None:
         """No-op."""