CI on pr only is not working on Azure DevOps Repo and Azure DevOps Pipeline

This code snippet is not working on Azure Repoes.

https://github.com/Azure/arc-cicd-demo-src/blob/master/.pipelines/az-vote-pr-pipeline.yaml#L3-L7

```
trigger: none
pr:
branches:
include:
- master
```

```
Git Source: Azure DevOps Repo
CI/CD: Azure DevOps Pipeline
```

Start with .pipelines/az-vote-pr-pipeline.yaml at lines 3-7 and review how the Azure DevOps Repo and Azure DevOps Pipeline interpret the trigger and pr settings. Reproduce the configuration in the linked example and determine whether pull-request-only CI runs; done means the expected PR trigger works for the master branch.

Written by the indexing model from the issue text.