fix: Add nil check for getOCIClientCertFromSecret

[joekr]

What this PR does / why we need it:
Adds a nil check around a possible nil case

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
NA

[joekr]

unit tests

        github.com/oracle/cluster-api-provider-oci              coverage: 0.0% of statements
ok      github.com/oracle/cluster-api-provider-oci/api/v1beta1  25.313s coverage: 23.3% of statements
ok      github.com/oracle/cluster-api-provider-oci/api/v1beta2  0.525s  coverage: 18.1% of statements
ok      github.com/oracle/cluster-api-provider-oci/cloud/config 0.615s  coverage: 86.1% of statements
        github.com/oracle/cluster-api-provider-oci/cloud/metrics                coverage: 0.0% of statements
ok      github.com/oracle/cluster-api-provider-oci/cloud/ociutil        0.796s  coverage: 15.9% of statements
ok      github.com/oracle/cluster-api-provider-oci/cloud/ociutil/ptr    1.784s  coverage: 100.0% of statements
ok      github.com/oracle/cluster-api-provider-oci/cloud/scope  21.571s coverage: 74.4% of statements
        github.com/oracle/cluster-api-provider-oci/cloud/scope/mocks            coverage: 0.0% of statements
        github.com/oracle/cluster-api-provider-oci/cloud/services/base          coverage: 0.0% of statements
        github.com/oracle/cluster-api-provider-oci/cloud/services/base/mock_base                coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/compute       [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/compute/mock_compute          coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/computemanagement     [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/computemanagement/mock_computemanagement              coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/containerengine       [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/containerengine/mock_containerengine          coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/identity      [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/identity/mock_identity                coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/loadbalancer  [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/loadbalancer/mock_lb          coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer   [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/networkloadbalancer/mock_nlb          coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/vcn   [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/vcn/mock_vcn          coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/cloud/services/workrequests  [no test files]
        github.com/oracle/cluster-api-provider-oci/cloud/services/workrequests/mock_workrequests                coverage: 0.0% of statements
ok      github.com/oracle/cluster-api-provider-oci/cloud/util   1.328s  coverage: 60.9% of statements
ok      github.com/oracle/cluster-api-provider-oci/controllers  3.323s  coverage: 59.2% of statements
ok      github.com/oracle/cluster-api-provider-oci/exp/api/v1beta1      7.721s  coverage: 15.3% of statements
ok      github.com/oracle/cluster-api-provider-oci/exp/api/v1beta2      1.817s  coverage: 5.8% of statements
ok      github.com/oracle/cluster-api-provider-oci/exp/controllers      2.146s  coverage: 56.3% of statements
        github.com/oracle/cluster-api-provider-oci/feature              coverage: 0.0% of statements
?       github.com/oracle/cluster-api-provider-oci/version      [no test files]

e2e tests

Ran 9 of 31 Specs in 3133.321 seconds
SUCCESS! -- 9 Passed | 0 Failed | 0 Pending | 22 Skipped

[joekr]

I think there might be a bug in this. I want to return nil, nil instead of error

I think this should work correctly. If the override is nil returning an error is what we want. Returning nil, nil breaks the contract of the function.