[release-1.12] Fix possible nil pointer dereferences on Consumer/ConsumerGroup

[knative-prow-robot]

This is an automated cherry-pick of #3631

[Cali0707]

/cc @creydr @pierDipi

[codecov]

Codecov Report

Attention: Patch coverage is 15.38462% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 48.75%. Comparing base (56b2bfc) to head (06a6a75).

Files	Patch %	Lines
control-plane/pkg/reconciler/consumer/consumer.go	20.00%	6 Missing and 2 partials ⚠️
...ls/kafka/eventing/v1alpha1/consumer_group_types.go	0.00%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-1.12    #3774      +/-   ##
================================================
- Coverage         48.78%   48.75%   -0.04%     
================================================
  Files               241      241              
  Lines             17422    17432      +10     
================================================
- Hits               8500     8499       -1     
- Misses             8219     8228       +9     
- Partials            703      705       +2     

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[creydr]

/lgtm

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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[pierDipi]

/retest-required