[Schema Inaccuracy] Check Run webhooks 'check_run' actually has nullable 'output'

# Schema Inaccuracy

Check Run webhooks claims output is non-null, but GitHub regularly sends data with this being null.

## Expected

'check_run' should have a nullable 'output'

## Reproduction Steps

I can get webhook payload examples if needed, but unlike REST API, this is a bit harder to reproduce at will.