[BUG] Cannot Create Document Monitors on 2.18

[rkbennett]

What is the bug?
When trying to create a per document monitor with the extraction query editor and using the default query with match_all, the following error is raised [alerting_exception] Can't get text on a START_OBJECT at 1:280

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Go to 'Alerting'
2. Click on 'Monitors'
3. Click 'Create monitor'
4. Select 'Per document monitor'
5. Use the 'Extraction query editor'
6. Keep the default query and click 'Create'

What is the expected behavior?
The monitor should create without error

What is your host/environment?

• OS: kubernetes 1.28 w/ opensearch images
• Version 2.18
• Plugins: Security, Dashboards