Description of problem: 

When using yarn export-dynamic command to wrap a third-party plugin, it expects app-config.yaml to be present with app.baseUrl config present in it. This is unnecessary as this file is not used by the wrapper at all and I can give any dummy value for this config and that's sufficient to make the yarn command happy. Here's the value I provided in this file to make the yarn command work:

app:
  baseUrl: "https://bogussite.com"

Prerequisites (if any, like setup, operators/versions):

Steps to Reproduce

1. Follow the instructions given at https://github.com/janus-idp/backstage-showcase/blob/main/showcase-docs/dynamic-plugins.md#frontend-plugins to create a dynamic wrapper for a third-party plugin. I ran into this issue when creating a wrapper for a third-party frontend plugin though it could be the same issue when wrapping a third-party backend plugin. 

Actual results:

When the app-config.yaml file is not present, the yarn export-dynamic command failed with the following error:

NotFoundError: Config file "<PROJECT_DIR>/app-config.yaml" does not exist

When an empty app-config.yaml file is present, the yarn export-dynamic command failed with the following error

Error: Missing required config value at 'app.baseUrl' in 'mock-config'

Expected results:

The yarn export-dynamic command should succeed and should not need app-config.yaml file to be present

Reproducibility (Always/Intermittent/Only Once):

Always

Build Details:

Additional info (Such as Logs, Screenshots, etc):