fix(argocd-apps): allow nullified top level item

[fxbtr]

Hi,

Let's say I have the following need, using Helmfile:

# default.values.yaml
applications:
  old-application:
    namespace: some-ns

# staging.values.yaml
# here I need old-application, same as default

# prod.values.yaml
# I no longer want old-application in this environment.
applications:
  old-application: null
  application:
    namespace: some-ns

That would make sense according to the helm documentation on deleting a default key.

But this was not doable in 2.0.3 due to

STDERR:
  coalesce.go:298: warning: cannot overwrite table with non table for argo.argocd-apps.applications.old-application (map[destination:map[namespace: server:https://kubernetes.default.svc] finalizers:[resources-finalizer.argocd.argoproj.io] namespace: project:applications source:map[directory:map[recurse:true] path: repoURL:... targetRevision:develop] syncPolicy:map[automated:map[prune:false selfHeal:false]]])
  coalesce.go:298: warning: cannot overwrite table with non table for argo.argocd-apps.applications.old-application (map[destination:map[namespace: server:https://kubernetes.default.svc] finalizers:[resources-finalizer.argocd.argoproj.io] namespace: project:applications source:map[directory:map[recurse:true] path: repoURL:... targetRevision:develop] syncPolicy:map[automated:map[prune:false selfHeal:false]]])
  Error: template: argo/charts/argocd-apps/templates/applications.yaml:6:19: executing "argo/charts/argocd-apps/templates/applications.yaml" at <$appData.additionalAnnotations>: nil pointer evaluating interface {}.additionalAnnotations
  Use --debug flag to render out invalid YAML

Signed-off-by: fxbtr

Checklist:

• I have bumped the chart version according to versioning
• I have updated the documentation according to documentation
• I have updated the chart changelog with all the changes that come with this pull request according to changelog.
• Any new values are backwards compatible and/or have sensible default.
• I have signed off all my commits as required by DCO.
• I have created a separate pull request for each chart according to pull requests
• My build is green (troubleshooting builds).

[mkilchhofer]

LGTM, thanks for your contribution @fxbtr 🙏