fix: kustomize render should workwith inline patches

[coperni]

Fixes: #9731

Description

Kustomize supports inline patches where the patch content is put inside the kustomization file as a single string. There is a bug in Skaffold where it will successfully render the Kustomization if the --set flag is not set.

Before

skaffold render -p dev --set="foo=bar"
read /Users/REDACTED/tmp/kustomize_example/overlays/prod: is a directory

After

$ skaffold render -p dev --set="foo=bar"
apiVersion: v1
data:
  foo: bar!
kind: ConfigMap
metadata:
  name: prod-basa
---
apiVersion: v1
kind: Pod
metadata:
  labels:
    app: myapp
  name: prod-myapp-pod
spec:
  containers:
    - image: nginx:123
      name: nginx