Set-AzAnalysisServicesServer cannot be used to disable firewall completely

## Description

Set-AzAnalysisServicesServer can be used to enable the firewall, or configure the rules, but it cannot be used to disable the firewall entirely.

## Steps to reproduce

```powershell
$FWConfigEmpty = New-AzAnalysisServicesFirewallConfig
Set-AzAnalysisServicesServer -Name $servername -ResourceGroupName $RGName -FirewallConfig $FWConfigEmpty
Get-AzAnalysisServicesServer -Name $servername -ResourceGroupName $RGName
```
The firewall will remain turned on, but the PBI service will be set to off

## Environment data

```
PS C:\WINDOWS\system32> $PSVersionTable

Name Value
---- -----
PSVersion 5.1.18362.628
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.18362.628
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
```

## Module versions

```powershell
ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 1.1.0 Az.AnalysisServices {Resume-AzAnalysisServicesServer, Suspend-AzAnalysisServicesServer, Get-AzAnalysisServicesServer, Remove-AzAnalysisServicesServer...}
```

## Debug output

```
No error. The firewall remains on, but the option to allow PowerBI Service is turned off.
```

## Error output

```
No error
```

Start with the Set-AzAnalysisServicesServer and New-AzAnalysisServicesFirewallConfig entry points and reproduce the behavior using the PowerShell commands in the issue. Compare the returned server state with the empty firewall configuration and inspect the associated implementation or tests. Done means an empty firewall configuration disables the firewall completely, rather than only turning off the Power BI service option.

Written by the indexing model from the issue text.