CosmosDB Account API should allow "userAssignedIdentities = null" when "identity.type" is "None"

Hi team,

We found that CosmosDB Account API in version 2023-04-15 doesn't allow "userAssignedIdentities = null" when "identity.type" is "None". And service API would fail with below error while creating CosmosDB Account with "userAssignedIdentities = null" and "identity.type = None". I assume the backend service should allow "userAssignedIdentities = null" when "identity.type" is "None".

Request payload:
```
PUT https://management.azure.com/subscriptions/{{subscriptionId}}/resourceGroups/acctestRG-cosmos-test10/providers/Microsoft.DocumentDB/databaseAccounts/acctest-ca-test10?api-version=2023-04-15

{
"identity": {
"type": "None",
"userAssignedIdentities": null
},
"kind": "GlobalDocumentDB",
"location": "westeurope",
"properties": {
"backupPolicy": null,
"capabilities": [],
"consistencyPolicy": {
"defaultConsistencyLevel": "Session",
"maxIntervalInSeconds": 5,
"maxStalenessPrefix": 100
},
"databaseAccountOfferType": "Standard",
"disableKeyBasedMetadataWriteAccess": false,
"disableLocalAuth": false,
"enableAnalyticalStorage": false,
"enableAutomaticFailover": false,
"enableFreeTier": false,
"enableMultipleWriteLocations": false,
"ipRules": [],
"isVirtualNetworkFilterEnabled": false,
"locations": [
{
"failoverPriority": 0,
"isZoneRedundant": false,
"locationName": "westeurope"
}
],
"networkAclBypass": "None",
"networkAclBypassResourceIds": [],
"publicNetworkAccess": "Enabled",
"virtualNetworkRules": []
},
"tags": {}
}
```

Error Message:
```
{
"message": "Object reference not set to an instance of an object.",
"code": "Internal Server Error"
}
```

No repository file, test, or entry point is named. Start by locating the CosmosDB 2023-04-15 account identity definition in the REST API specifications and checking how null is represented; done means the intended handling of userAssignedIdentities when identity.type is None is reflected in the API contract and its relevant validation path.

Written by the indexing model from the issue text.