[release-1.13] Avoid possible nil pointer dereferences in consumergroup scheduling

[knative-prow-robot]

This is an automated cherry-pick of #3775

[codecov]

Codecov Report

Attention: Patch coverage is 53.33333% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 52.43%. Comparing base (c9a7165) to head (de56581).

Files	Patch %	Lines
...lane/pkg/reconciler/consumergroup/consumergroup.go	57.14%	4 Missing and 2 partials ⚠️
...l-plane/pkg/reconciler/consumergroup/controller.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##             release-1.13    #3785      +/-   ##
==================================================
+ Coverage           48.41%   52.43%   +4.01%     
- Complexity              0      845     +845     
==================================================
  Files                 241      338      +97     
  Lines               17545    20759    +3214     
  Branches                0      273     +273     
==================================================
+ Hits                 8494    10884    +2390     
- Misses               8337     8993     +656     
- Partials              714      882     +168     

Flag	Coverage Δ
java-unittests	74.36% <ø> (?)

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[Cali0707]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Cali0707, knative-prow-robot

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[Cali0707]

/retest-required

[matzew]

/test reconciler-tests