fix(core): better error message for empty var names (#32073)

Previously, we hit an index out of range error with empty variable names
(accessing tag[0]), now we through a slightly nicer error

---------

Co-authored-by: Mason Daugherty <mason@langchain.dev>

libs/core/tests/unit_tests/prompts/test_structured.py

@@ -2,6 +2,7 @@ from functools import partial
 from inspect import isclass
 from typing import Any, Union, cast
 
+import pytest
 from pydantic import BaseModel
 
 from langchain_core.language_models import FakeListChatModel
@@ -10,6 +11,7 @@ from langchain_core.load.load import loads
 from langchain_core.messages import HumanMessage
 from langchain_core.prompts.structured import StructuredPrompt
 from langchain_core.runnables.base import Runnable, RunnableLambda
+from langchain_core.utils.mustache import ChevronError
 from langchain_core.utils.pydantic import is_basemodel_subclass
 
 
@@ -128,3 +130,8 @@ def test_structured_prompt_template_format() -> None:
     assert prompt.invoke({"person": {"name": "foo"}}).to_messages() == [
         HumanMessage("hi foo")
     ]
+
+
+def test_structured_prompt_template_empty_vars() -> None:
+    with pytest.raises(ChevronError, match="empty tag"):
+        StructuredPrompt([("human", "hi {{}}")], schema={}, template_format="mustache")