Bug: OpenAIFunctionsAgentOutputParser doesn't handle functions with no args (#13467)

**Description/Issue:** 
When OpenAI calls a function with no args, the args are `""` rather than
`"{}"`. Then `json.loads("")` blows up. This PR handles it correctly.

**Dependencies:** None

libs/langchain/langchain/agents/output_parsers/openai_functions.py

@@ -44,7 +44,12 @@ class OpenAIFunctionsAgentOutputParser(AgentOutputParser):
         if function_call:
             function_name = function_call["name"]
             try:
-                _tool_input = json.loads(function_call["arguments"])
+                if len(function_call["arguments"].strip()) == 0:
+                    # OpenAI returns an empty string for functions containing no args
+                    _tool_input = {}
+                else:
+                    # otherwise it returns a json object
+                    _tool_input = json.loads(function_call["arguments"])
             except JSONDecodeError:
                 raise OutputParserException(
                     f"Could not parse tool input: {function_call} because "

libs/langchain/tests/unit_tests/agents/output_parsers/test_openai_functions.py

@@ -46,6 +46,22 @@ def test_func_call() -> None:
     assert result.message_log == [msg]
 
 
+# Test: Model response with a function call for a function taking no arguments
+def test_func_call_no_args() -> None:
+    parser = OpenAIFunctionsAgentOutputParser()
+    msg = AIMessage(
+        content="LLM thoughts.",
+        additional_kwargs={"function_call": {"name": "foo", "arguments": ""}},
+    )
+    result = parser.invoke(msg)
+
+    assert isinstance(result, AgentActionMessageLog)
+    assert result.tool == "foo"
+    assert result.tool_input == {}
+    assert result.log == ("\nInvoking: `foo` with `{}`\nresponded: LLM thoughts.\n\n")
+    assert result.message_log == [msg]
+
+
 # Test: Model response with a function call (old style tools).
 def test_func_call_oldstyle() -> None:
     parser = OpenAIFunctionsAgentOutputParser()