fix: GenerateDatastoreMapForBlockVolumes fails if there are no entities

[Mattes83]

What this PR does / why we need it:
We have a multi vcenter setup in which all datastores are currently in only one vcenter.
This leads to many errors like the following:
{"level":"error","time":"2023-06-22T11:14:01.916433945Z","caller":"common/authmanager.go:247","msg":"failed to get datastores with required priv for vCenter \"xxx\". Error: ServerFaultCode: \nRequired parameter entities is missing\n\nwhile parsing call information for method HasUserPrivilegeOnEntities\nat line 2, column 66\n\nwhile parsing SOAP body\nat line 2, column 60\n\nwhile parsing SOAP envelope\nat line 2, column 0\n\nwhile parsing HTTP request for method hasUserPrivilegeOnEntities\non object of type vim.AuthorizationManager\nat line 1, column 0","TraceId":"2b45f0db-1d50-4299-9f15-f6a34f8807ec","stacktrace":"sigs.k8s.io/vsphere-csi-driver/v3/pkg/csi/service/common.GenerateDatastoreMapForBlockVolumes\n\t/build/pkg/csi/service/common/authmanager.go:247\nsigs.k8s.io/vsphere-csi-driver/v3/pkg/csi/service/common.(*AuthManager).refreshDatastoreMapForBlockVolumes\n\t/build/pkg/csi/service/common/authmanager.go:158\nsigs.k8s.io/vsphere-csi-driver/v3/pkg/csi/service/common.ComputeDatastoreMapForBlockVolumes\n\t/build/pkg/csi/service/common/authmanager.go:199"} {"level":"warn","time":"2023-06-22T11:14:01.916459756Z","caller":"common/authmanager.go:166","msg":"auth manager: failed to get updated datastoreMapForBlockVolumes for vCenter \"xxx\", Err: ServerFaultCode: \nRequired parameter entities is missing\n\nwhile parsing call information for method HasUserPrivilegeOnEntities\nat line 2, column 66\n\nwhile parsing SOAP body\nat line 2, column 60\n\nwhile parsing SOAP envelope\nat line 2, column 0\n\nwhile parsing HTTP request for method hasUserPrivilegeOnEntities\non object of type vim.AuthorizationManager\nat line 1, column 0","TraceId":"2b45f0db-1d50-4299-9f15-f6a34f8807ec"}

We also suspect this to be responsible for the need to restart the csi vsphere controller from time to time to get volumes provisioned.

Testing done:
Not sure if I need to write a test for this. If yes i'd appreciate a hint on where to add it.
Special notes for your reviewer:

Release note:

[k8s-ci-robot]

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[divyenpatel]

/ok-to-test

[shalini-b]

/approve

[divyenpatel]

@Mattes83 we are trying to understand the usecase. So you have just added another vCenter in the config secret and at later point of time, planning to add nodes and datastores?

[Mattes83]

Yes, exactly. We are currently in the process of migrating clusters from one vcenter to another. In between we have nodes in multiple vcenters.
At the beginning and at the end of the migration we have credentials for both vcenters inside the cluster, but nodes and datastores are only in one vcenter.
Of course this can be solved by removing the not needed config, but there may be a timeframe in which there are credentials for unused vcenters

[divyenpatel]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: divyenpatel, Mattes83, shalini-b

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