Make agent move to actual random closest position

[EwoutH]

This fixes a bug in which move_agent_to_one_of is deterministic when pos has multiple closest choices and selection="closest".

Closes #2117.

[EwoutH]

I don't understand this error:

        # Only move agent if there are positions given (non-empty list)
        if pos:
            if selection == "random":
                chosen_pos = agent.random.choice(pos)
            elif selection == "closest":
                current_pos = agent.pos
                # Find the closest position without sorting all positions
                closest_pos = None
                min_distance = float("inf")
>               for p in agent.random.shuffle(pos):
E               TypeError: 'NoneType' object is not iterable

The if pos: should filter out None values.

[EwoutH]

Right, random.shuffle does not return any value; it modifies the list pos in-place. Therefore, agent.random.shuffle(pos) results in None, and trying to iterate over None leads to the error above.

By putting it on a separate line above it now works as expected.

[rht]

Make sure to delete the branch after the PR has been merged.