fix: openkruise health check npe error #19545

[alswl]

Fixes ISSUE #19545

Reproduce this case in unit test TestLuaHealthScript with commit 9b0e6a4:

2024-09-08T05:53:13.5111087Z time="2024-09-08T05:52:49Z" level=fatal msg="<string>:12: cannot perform sub operation between number and nil\nstack traceback:\n\t<string>:12: in main chunk\n\t[G]: ?"

The obj.spec.updateStrategy.rollingUpdate.partition missing null value check, and the fix is in next commit.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.81%. Comparing base (4736657) to head (f468807).
Report is 30 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19660      +/-   ##
==========================================
+ Coverage   55.80%   55.81%   +0.01%     
==========================================
  Files         320      320              
  Lines       44250    44281      +31     
==========================================
+ Hits        24693    24717      +24     
- Misses      16998    17006       +8     
+ Partials     2559     2558       -1     

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[alswl]

@christianh814 please review this pull-request.