Amplify Console doesn't ask for IAM role for project with custom backends

**Describe the bug**
When using a custom backend (e.g. SAM) defined in the amplify.yml file (see [this sample project](https://github.com/aws-samples/aws-dog-no-dog/blob/complete/amplify.yml)), Amplify Console doesn't ask for an IAM role to deploy the backend, which causes the build phase to fail.

**To Reproduce**
1. Go to Amplify Console
2. Deploy this sample application using the 'complete' branch: https://github.com/aws-samples/aws-dog-no-dog
3. See that Amplify Console doesn't ask for an IAM role.
4. Wait until the deployment fails in the 'build' phase.

**Expected behavior**
When there is a "backend" section in the amplify.yml file, Amplify Console should ask for an IAM role. Alternatively, Amplify Console should always ask for an IAM role.

**Additional context**
None.

**Sample code**
See https://github.com/aws-samples/aws-dog-no-dog/tree/complete

Start with the sample project's amplify.yml on the complete branch and reproduce the deployment in Amplify Console, watching the build phase where it fails. Done means a project with a backend section prompts for an IAM role, and the backend deployment proceeds successfully.

Written by the indexing model from the issue text.