[Fleet] Unable to save yaml multi: true field in Custom AWS Logs

[jen-huang]

1. Force install aws_logs-1.3.0:

POST kbn:/api/fleet/epm/packages/aws_logs-1.3.0
{
  "force": true
}

2. Go to Add integration for Custom AWS Logs package
3. Enable Collect Logs from S3 Bucket and expand Advanced options
4. Fill in value for File Selectors, observe error, error persists when adding a new row as well:

5. Observe that creating the policy is disabled. It only renables after File Selectors is completely cleared. This file is defined as type: yaml and multi: true

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)