Add more nil pointer check for CSI related code in backup controller

[blackpiglet]

Add some corner cases checking for CSI snapshot in backup controller.

Signed-off-by: Xun Jiang [email protected]

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Does your change fix a particular issue?

Fixes https://github.com/vmware-tanzu/velero/issues/5207 Fixes https://github.com/vmware-tanzu/velero/issues/5244

Please indicate you've done the following:

• [ ] 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-commenter]

Codecov Report

Merging #5388 (027cba1) into main (66f6365) will increase coverage by 0.20%. The diff coverage is 9.52%.

@@            Coverage Diff             @@
##             main    #5388      +/-   ##
==========================================
+ Coverage   40.56%   40.76%   +0.20%     
==========================================
  Files         236      238       +2     
  Lines       20385    20446      +61     
==========================================
+ Hits         8269     8335      +66     
+ Misses      11511    11500      -11     
- Partials      605      611       +6     

Impacted Files	Coverage Δ
pkg/builder/volume_snapshot_builder.go	0.00% <0.00%> (ø)
pkg/builder/volume_snapshot_content_builder.go	0.00% <0.00%> (ø)
pkg/controller/backup_controller.go	56.52% <42.85%> (+8.36%)	:arrow_up:

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[reasonerjt]

@blackpiglet Please combine all the nil-pointer checking in this PR and make sure this is cherry-picked to v1.9.3

[blackpiglet]

Tried to add a UT case for function waitVolumeSnapshotReadyToUse, but didn't make the VolumeSnapshot informer Lister work in UT, so modified according to Yonghui's suggestion. Will add the UT case later.