Fix run_async RuntimeError

[mike1936]

Description

run_async helper function raising RuntimeError sometimes when the event_loop is None.
My working case for this RuntimeError to occur is calling the run_async function inside a prefect.flow, which is just a wrapped function provided by another package named prefect for workflow orchestration.

Motivation and Context

RuntimeError raised by asyncio.get_event_loop not properly dealt by run_async function.
The event_loop is newly generated if get None then set to asyncio's event_loop

How Has This Been Tested?

NA

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[waketzheng]

How to reproduce your bug?

And how about change run_async to be as follows:

def run_async(coro: Coroutine) -> None:
    async def runner():
        try:
            await coro
        finally:
            await connections.close_all(discard=True)

    asyncio.run(runner())

[abondar]

If it is actual error please describe it in issue with steps to reproduce