Harrison/error zero tools (#6340)

Co-authored-by: Juhee Kim <46583939+juppytt@users.noreply.github.com>

tests/unit_tests/agents/test_serialization.py

@@ -3,12 +3,19 @@ from tempfile import TemporaryDirectory
 
 from langchain.agents.agent_types import AgentType
 from langchain.agents.initialize import initialize_agent, load_agent
+from langchain.agents.tools import Tool
 from langchain.llms.fake import FakeListLLM
 
 
 def test_mrkl_serialization() -> None:
     agent = initialize_agent(
-        [],
+        [
+            Tool(
+                name="Test tool",
+                func=lambda x: x,
+                description="Test description",
+            )
+        ],
         FakeListLLM(responses=[]),
         agent=AgentType.ZERO_SHOT_REACT_DESCRIPTION,
         verbose=True,