fix an edge case bug that "identity_policies" is nil

[tianhaopx]

Fixes #17006

client side: check if "identity_policies" is nil, goto DONE, then cli vault login will not return unexpected error.

server side: check if key out.NamespaceID exists in the map identityPolicies returned from func fetchEntityAndDerivedPolicies

[hashicorp-cla]

All committers have signed the CLA.

[peteski22]

Hi @tianhaopx, thanks for the submission. It looks good, I just wonder if you might be able to add a test to secret_test.go to check the return from TokenPolicies? I think we're good to merge it in after that. :)