Fix: fix panic when workflow is skipped

[FogDong]

Signed-off-by: FogDong dongtianxin.tx@alibaba-inc.com

Description of your changes

Fixes #

I have:

• Read and followed KubeVela's contribution process.
• Related Docs updated properly. In a new feature or configuration option, an update to the documentation is necessary.
• Run make reviewable to ensure this PR is ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested

Special notes for your reviewer

[codecov]

Codecov Report

Base: 65.13% // Head: 65.13% // No change to project coverage 👍

Coverage data is based on head (b3beb58) compared to base (e8f00ce).
Patch coverage: 33.33% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #122   +/-   ##
=======================================
  Coverage   65.13%   65.13%           
=======================================
  Files          41       41           
  Lines        4228     4228           
=======================================
  Hits         2754     2754           
  Misses       1108     1108           
  Partials      366      366           

Flag	Coverage Δ
unit-test	65.13% <33.33%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/executor/workflow.go	78.92% <33.33%> (ø)

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[github-actions]

Successfully created backport PR #123 for release-0.4.

[github-actions]

Backport failed for release-0.3, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally.

git fetch origin release-0.3
git worktree add -d .worktree/backport-122-to-release-0.3 origin/release-0.3
cd .worktree/backport-122-to-release-0.3
git checkout -b backport-122-to-release-0.3
ancref=$(git merge-base e8f00ceab29d2f6f9034bab98655eef96e4f7374 b3beb580a69b26b084f9494649bf37e3626682e0)
git cherry-pick -x $ancref..b3beb580a69b26b084f9494649bf37e3626682e0