Fix issue with BatchedStepResult with no agents (#3240)

ml-agents/mlagents/trainers/agent_processor.py

 import sys
-import numpy as np
 from typing import List, Dict, Deque, TypeVar, Generic
 from collections import defaultdict, Counter, deque
 
         for global_id in action_global_agent_ids:
             self.last_take_action_outputs[global_id] = take_action_outputs
 
-        for _id in np.nditer(batched_step_result.agent_id):  # Explicit numpy iteration
+        for _id in batched_step_result.agent_id:  # Assume agent_id is 1-D
             local_id = int(
                 _id
             )  # Needed for mypy to pass since ndarray has no content type

ml-agents/mlagents/trainers/tests/test_agent_processor.py

     # Assert that the AgentProcessor is empty
     assert len(processor.experience_buffers[0]) == 0
 
+    # Test empty BatchedStepResult
+    mock_step = mb.create_mock_batchedstep(
+        num_agents=0,
+        num_vector_observations=8,
+        action_shape=[2],
+        num_vis_observations=num_vis_obs,
+    )
+    processor.add_experiences(mock_step, 0, ActionInfo([], [], {}, []))
+    # Assert that the AgentProcessor is still empty
+    assert len(processor.experience_buffers[0]) == 0
+
 
 def test_agent_manager():
     policy = create_mock_policy()