[BUG] Mongo Cluster API should allow `userAssignedIdentities=null` when `identity` is `None`

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/1653285537c3f05521c480112d2a36645878c274/specification/mongocluster/resource-manager/Microsoft.DocumentDB/MongoCluster/stable/2025-09-01/mongoCluster.json#L2034

### API Spec version

2025-09-01

### Describe the bug

According to below repro steps, I am trying to update the existing mongo cluster with “identity = None” but service API failed with below error. It seems that the error message indicates that service API doesn’t allow userAssignedIdentities=null when identity is None. Could you take a look this issue. Thanks.

### Expected behavior

The existing mongo cluster should allow userAssignedIdentities=null when identity is None.

### Actual behavior

Service API returns below error while updating the existing mongo cluster with "identity=None" and "userAssignedIdentities=null" is set in the request payload.

### Reproduction Steps

Repro steps:
1. Create mongo cluster without “identity”
```
PUT https://management.azure.com/subscriptions/xx-xx-xx-xx-xx/resourceGroups/acctestRG-xxx/providers/Microsoft.DocumentDB/mongoClusters/acctest-xxx

{
"location": "centralus",
"properties": {
"administrator": {
"password": "xxxx",
"userName": "adminTerraform"
},
"compute": {
"tier": "Free"
},
"createMode": "Default",
"highAvailability": {
"targetMode": "Disabled"
},
"publicNetworkAccess": "Enabled",
"serverVersion": "7.0",
"sharding": {
"shardCount": 1
},
"storage": {
"sizeGb": 32
}
}
}
```

2. Update the existing mongo cluster with “identity = None” and "userAssignedIdentities=null"
```
PUT https://management.azure.com/subscriptions/xx-xx-xx-xx-xx/resourceGroups/acctestRG-xxx/providers/Microsoft.DocumentDB/mongoClusters/acctest-xx

{
"id": "/subscriptions/xx-xx-xxx-xxx-xx/resourceGroups/acctestRG-xx/providers/Microsoft.DocumentDB/mongoClusters/acctest-xxx",
"identity": {
"type": "None",
"userAssignedIdentities": null
},
"location": "centralus",
"name": "acctest-xxx",
"properties": {
"administrator": {
"userName": "adminTerraform"
},
"authConfig": {
"allowedModes": [
"NativeAuth"
]
},
"backup": {
"earliestRestoreTime": "2025-11-03T05:32:28Z"
},
"clusterStatus": "Ready",
"compute": {
"tier": "Free"
},
"connectionString": "xxxxxx",
"createMode": "Default",
"dataApi": {
"mode": "Disabled"
},
"highAvailability": {
"targetMode": "Disabled"
},
"infrastructureVersion": "2.0",
"privateEndpointConnections": [],
"provisioningState": "Succeeded",
"publicNetworkAccess": "Enabled",
"replica": {
"replicationState": "Active",
"role": "Primary"
},
"serverVersion": "7.0",
"sharding": {
"shardCount": 1
},
"storage": {
"sizeGb": 32,
"type": "PremiumSSD"
}
},
"tags": {
"env": "test"
},
"type": "Microsoft.DocumentDB/mongoClusters"
}
```

3. API returns the following error.
```
Error: updating Mongo Cluster (Subscription: "xx-xx-xx-xxx-xx"
Resource Group Name: "acctestRG-xxx"
Mongo Cluster Name: "acctest-xxx"): performing CreateOrUpdate: unexpected status 400 (400 Bad Request) with error: bad_request: Found schema errors for api-version 2025-09-01. Errors: The property '#/identity/userAssignedIdentities' of type null did not match the following type: object in schema xx-xx-xx-xx-xxx
The property '#/identity' of type object did not match one or more of the required schemas. The schema specific errors were:

- anyOf #0:
- The property '#/identity/userAssignedIdentities' of type null did not match the following type: object
- The property '#/identity/type' value "None" did not match one of the following values: UserAssigned
- anyOf #1:
- The property '#/identity' of type object matched the disallowed schema
- The property '#/identity' contained undefined properties: 'userAssignedIdentities'
```

### Environment

PROD

Start at specification/mongocluster/resource-manager/Microsoft.DocumentDB/MongoCluster/stable/2025-09-01/mongoCluster.json around line 2034 and inspect the identity schema. Compare the None and UserAssigned cases with the reproduction payload and service error. Done means the API specification permits userAssignedIdentities=null when identity.type is None, with the schema still rejecting invalid identity combinations.

Written by the indexing model from the issue text.