configuration_overrides shouldn't be required

`configuration_overrides` is a required field in `EmrServerlessStartJobOperator`, but [it's not a required field](https://docs.aws.amazon.com/emr-serverless/latest/APIReference/API_StartJobRun.html#API_StartJobRun_RequestBody).

Error message when trying to start a job without it:

```
E airflow.exceptions.AirflowException: Argument ['configuration_overrides'] is required
```

Start at the EmrServerlessStartJobOperator implementation and trace the validation that marks configuration_overrides as required. Compare it with the EMR Serverless StartJobRun request documentation, then verify that starting a job without configuration_overrides no longer raises the reported exception.

Written by the indexing model from the issue text.