DSOS-2759: allow default max capacity to be set

[drobinson-moj]

Fix bug - allow default max capacity to be set by allowing null value for max_group_prepared_capacity. This is better than setting to max size
Tested in nomis account - fixes the issue

[github-actions]

Trivy Scan Success

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</details> #### `Checkov Scan` Success
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CTFLint Scan Success

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Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.5.0)
tflint will check the following folders:

Trivy Scan Success

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Trivy will check the following folders: