Unalbe to set Storage_URI to null once enabled on boot_diagnostics on VM

Here is the scenario,
1.User creates a VM with boot_diagnostics enabled
2.User disables boot_diagnostics and deletes the Storage Account.
3.When we send the following json blob in the VM payload we expect the server to update the Storage_URI to null.
"diagnosticsProfile": {
"bootDiagnostics": {
"enabled": false,
"storageUri": ""
}
},
4.Even though the boot_diagnostics is disabled, the storage_uri variable still has the old value which is returned to the user in the GET api. This is an issue and needs to be fixed.

The issue identifies the VM payload's diagnosticsProfile.bootDiagnostics fields; start by tracing the REST API definition and update handling for enabled=false with an empty storageUri. Verify that after the Storage Account is deleted, the update response and subsequent GET no longer return the old storageUri value.

Written by the indexing model from the issue text.