Check whether the volume's source is PVC before fetching its PV.

[blackpiglet]

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Does your change fix a particular issue?

Fixes #7929

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• Accepted the DCO. Commits without the DCO will delay acceptance.
• Created a changelog file or added /kind changelog-not-required as a comment on this pull request.
• Updated the corresponding documentation in site/content/docs/main.

[codecov]

Codecov Report

Attention: Patch coverage is 46.15385% with 7 lines in your changes missing coverage. Please review.

Project coverage is 58.83%. Comparing base (28d64c2) to head (d89a9f7).

Files	Patch %	Lines
internal/volumehelper/volume_policy_helper.go	46.15%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7967   +/-   ##
=======================================
  Coverage   58.83%   58.83%           
=======================================
  Files         345      345           
  Lines       28795    28798    +3     
=======================================
+ Hits        16941    16944    +3     
  Misses      10425    10425           
  Partials     1429     1429           

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