Fix type hint (#701)

Author: jaagut

bitbots_behavior/bitbots_blackboard/bitbots_blackboard/capsules/costmap_capsule.py

@@ -47,7 +47,9 @@ def __init__(self, node, blackboard):
 
         self.base_costmap: Optional[np.ndarray] = None  # generated once in constructor field features
         self.costmap: Optional[np.ndarray] = None  # updated on the fly based on the base_costmap
-        self.gradient_map: Optional[np.ndarray] = None  # global heading map (static) only dependent on field structure
+        self.gradient_map: Optional[list[np.ndarray]] = (
+            None  # global heading map (static) only dependent on field structure
+        )
 
         # Calculates the base costmap and gradient map based on it
         self.calc_base_costmap()