Set default empty object to with field

[ffjlabo]

What this PR does:

as title

Why we need it:

Currently, the pipeline with stages that don't have a field is failed because it is failing to parse the empty stage config.
(K8S_PRIMARY_ROLLOUT is failed in the example below)

apiVersion: pipecd.dev/v1beta1
kind: Application
spec:
  name: canary
  labels:
    env: example
    team: product
  pipeline:
    stages:
      # Deploy the workloads of CANARY variant. In this case, the number of
      # workload replicas of CANARY variant is 10% of the replicas number of PRIMARY variant.
      - name: K8S_CANARY_ROLLOUT
        with:
          replicas: 10%
      # Wait 10 seconds before going to the next stage.
      - name: WAIT
        with:
          duration: 10s
      # Update the workload of PRIMARY variant to the new version.
      - name: K8S_PRIMARY_ROLLOUT
      # Destroy all workloads of CANARY variant.
      - name: K8S_CANARY_CLEAN

Which issue(s) this PR fixes:

Part of #5530

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.46%. Comparing base (d73300d) to head (7c04d12).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5943      +/-   ##
==========================================
- Coverage   28.47%   28.46%   -0.01%     
==========================================
  Files         520      520              
  Lines       56415    56402      -13     
==========================================
- Hits        16063    16056       -7     
+ Misses      39086    39080       -6     
  Partials     1266     1266              

Flag	Coverage Δ
.	23.76% <ø> (+0.01%)	⬆️
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	48.86% <ø> (-0.37%)	⬇️
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[t-kikuc]

nice

[Warashi]

Nice 👍🏻