fix: handle empty kustomize bases

[cbodonnell]

What this PR does / why we need it:

Follow-up to #3996

This PR fixes an issue where kustomize build could fail for KOTS instances that were upgraded from older versions before the rendered directory was added. This can occur because previous application archives that don't contain the rendered directory are rendered on-the-fly when diffing. It's possible that some of these older application archives contain kustomization.yaml files that are considered "empty", which is no longer supported by the latest kustomize version (>= 5.1.x).

This PR introduces a logic before kustomize build is run to check if a base kustomization is empty. If it's empty, it will be modified to include the kots.io/kustomization=base label, which causes kustomize to no longer interpret it as empty according to this check.

Which issue(s) this PR fixes:

SC-88587

Special notes for your reviewer:

Considered skipping kustomize build all together if the base was empty, but then any midstream or downstream resources would not be added (pull secrets for example).

Steps to reproduce

1. Install KOTS 1.95 or earlier (pre rendered directory)
2. Deploy an application that results in empty kustomize bases. Bitnami helm charts containing the common dependency can result in this.
3. Upgrade KOTS to the latest version
4. Try to deploy a new version or diff between new and old versions

Does this PR introduce a user-facing change?

Fixes an issue where deployments or diffing could fail after upgrading from KOTS 1.95 or earlier if versions contained empty Kustomize bases

Does this PR require documentation?

NONE