[BUG] Export gets segmentation violation if access to cluster resources is forbidden

[dmartinol]

What version of crane are you running, and what are your clutsters+platform
Commit #149
Running export with the new option --cluster-scoped-rbac on cluster where the configured user has no admin role

What did you expect to happen?
To export only the accessible resources

What actually happened?
segmentation violation happened

Please include any relevant logs or errors

INFO[0043] Filtering for kind ClusterRole (1008 found)  
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xe0 pc=0x10110cfe1]

goroutine 1 [running]:
github.com/konveyor/crane/cmd/export.(*ClusterScopedRbacHandler).acceptClusterRole(0xc000565968, {0xc000ee0ba0})

[github-actions]

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