Fix: applied default cluster to created target

[zuoningz]

Description of your changes

In CreateTarget, a default cluster should be created if cluster is not speicified in the request. However, cluster is only applied to the request variable, not the actual created target . With this fix, the created target would have default cluster

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 yarn lint to ensure the frontend changes are ready for review.
• Run make reviewableto ensure the server changes are ready for review.
• Added backport release-x.y labels to auto-backport this PR if necessary.

Special notes for your reviewer

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09 🎉

Comparison is base (8a81aa1) 58.79% compared to head (9ec82ca) 58.89%.

❗ Current head 9ec82ca differs from pull request most recent head 197224d. Consider uploading reports for the commit 197224d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #857      +/-   ##
==========================================
+ Coverage   58.79%   58.89%   +0.09%     
==========================================
  Files         114      114              
  Lines       19822    19822              
==========================================
+ Hits        11655    11674      +19     
+ Misses       6720     6706      -14     
+ Partials     1447     1442       -5     

Flag	Coverage Δ
apiserver-unittests	32.05% <100.00%> (-0.02%)	⬇️
server-e2e-tests	48.37% <100.00%> (+0.16%)	⬆️

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

Impacted Files	Coverage Δ
pkg/server/domain/service/target.go	56.66% <100.00%> (-1.67%)	⬇️

... and 6 files with indirect coverage changes

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