AWS cloud formation - revisit null checks

[kat-ms]

⚠️ Does this change require a corresponding Server Change?
⚠️ If so - please add a "Requires Server Change" label to this PR!

A previous PR fixed the null reference exception reported by the customer. However the impact of AWS v4 of the SDK returning null collections is wider spread than just the file that was targeted.

This is another attempt to cover off the remaining bases:

• In Program.cs have set Amazon.AWSConfigs.InitializeCollections = true; - which restores v3 collection behaviour where empty collections are returned instead of null (ref: https://docs.aws.amazon.com/sdk-for-net/v4/developer-guide/net-dg-v4.html)
• Where identified (with the help of Claude) targeted null checks have been added to collections where they may be null.
• AwsKubernetesDiscoverer and S3PackageDownloader have been left untested as they directly instantiate the SDK client (would require refactoring)

Fixes FD-273