Ignore empty autoscaling config ARNs

[odlp]

Seems like my earlier PR (#33) has broken the action for users who don't want to specify an auto-scaling-config-arn input. I didn't realise @actions/core getInput will return an empty string if a value isn't set, rather than undefined.

Issue #, if available: https://github.com/awslabs/amazon-app-runner-deploy/issues/36

Description of changes:

• Introduces a more specific test for getConfig, because exercising this through index.test.ts is quite involved
• Introduces a new getOptionalInputStr helper in action-configuration, which returns string | undefined

Notes

The tests for this PR adjust the process environment to specify inputs (mimicking the environment as it would be while the action is running). This approach could also be used in index.test.ts too, as a replacement for FakeInput. This might make the tests simpler (let me know if you'd like a contribution adopting this approach).

Here's the reference for GitHub Actions inputs where I discovered it sets inputs as environment variables, which is what @actions/core getInput ends up reading:

When you specify an input in a workflow file or use a default input value, GitHub creates an environment variable for the input with the name INPUT_. The environment variable created converts input names to uppercase letters and replaces spaces with _ characters.

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