storage: API incorrectly returns an error when User Assigned Identities are specified

Service: `Storage`
API Version `2021-04-01`

The Storage API supports Managed Identity however returns an error when an empty list of User Assigned Identities is provided when specifying a a System Assigned Identity.

Since a System Assigned Identity doesn't use any of the User Assigned Identities and there are none specified, these should be being ignored - however instead the API returns:

> Failed to perform resource identity operation. Status: 'BadRequest'. Response: '{\"error\":{\"code\":\"BadRequest\",\"message\":\"The request format was unexpected, a non-UserAssigned identity type should not contain: userAssignedIdentities\"}}

Can the Storage API be updated to fix this bug?

Thanks!

No file or test is named. Start by locating the Storage API 2021-04-01 identity schema and its validation coverage in the REST API specifications; confirm how system-assigned identity requests represent an empty user-assigned identity list. Done means the specification accepts this request shape and repository validation passes.

Written by the indexing model from the issue text.