bug: create_event raises IndexError for empty string values

[aerosta]

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Python version (python --version)

Python 3.11.2

Operating system/version

macOS 26.2

NeMo-Guardrails version (if you must use a specific version and not the latest

No response

Describe the bug

create_event in nemoguardrails/actions/core.py checks v[0] == "$" after confirming v is a string. When v is an empty string (""), this raises IndexError: string index out of range.

This looks like an edge case in the $ prefix check for string values. Similar checks in other parts of the codebase use .startswith("$"), which handles empty strings safely.

Steps To Reproduce

import asyncio
from nemoguardrails.actions.core import create_event

asyncio.run(create_event(event={"_type": "SomeEvent", "param": ""}))

Expected Behavior

Empty string values should pass through unchanged without raising an error.

Actual Behavior

create_event raises IndexError: string index out of range when an event contains an empty string value.