Assign NULL in deploy.helm.releases.[0].setValueTemplates

### Expected behavior
I wanted to use `skaffold dev`, after temporarily removing some defaults in the Helm template.
(e.g. `nodeSelector`)

In the skaffold.yaml below,
Expected behavior is that the nodeSelector statement is deleted, when `null` or `{}` is assigned to `deploy.helm.releases.[0].setValueTemplates.nodeSelector`.

### Actual behavior
`"null"`, `""` works correctly
`null`, `{}`, `"{}"` doesn't work

### Information

- Skaffold version: v2.6.0-1-gb25a5ed52
- Operating system: Ubuntu 22 LTS
- Installed via: Cloud Code
- Contents of skaffold.yaml:
```yaml
# my helm template
(...)

{{- with .Values.nodeSelector }}
nodeSelector:
{{- toYaml . | nindent 8 }}
{{- end }}

(...)
```

```yaml
# my default helm values
(...)
nodeSelector:
my-node: worker
(...)
```
```yaml
# skaffold.yaml
(...)
- name: alpha
patches:
- op: add
path: /deploy/helm/releases
value:
- name: my-release
(...)
setValueTemplates:
nodeSelector: "null" # PASS: "null", "" # FAIL: {}, "{}", null
(...)
```

### P.S.
- Is this the original intended feature of skaffold?
- If you know related docs or issue numbers, please let me know.
Thank you