Fix nil panic in cloneset validating webhook

[Spground]

Ⅰ. Describe what this PR does

Fix potential nil panic in CloneSet validating webhook when Pod's controller owner ref is nil.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 48.81%. Comparing base (0d0031a) to head (be5142f).
Report is 58 commits behind head on master.

Files	Patch %	Lines
pkg/webhook/cloneset/validating/validation.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1678      +/-   ##
==========================================
+ Coverage   47.91%   48.81%   +0.90%     
==========================================
  Files         162      188      +26     
  Lines       23491    19278    -4213     
==========================================
- Hits        11256     9411    -1845     
+ Misses      11014     8636    -2378     
- Partials     1221     1231      +10     

Flag	Coverage Δ
unittests	48.81% <0.00%> (+0.90%)	⬆️

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[ABNER-1]

/lgtm

[furykerry]

/lgtm
/approve

[kruise-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: furykerry

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