[Bug Report]: Resource Group modules for Managedby

### Describe the bug

When updating from the old resources modules, the below code set the managedBy to ''

```
@description('Optional. The ID of the resource that manages this resource group.')
param managedBy string = ''

resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
managedBy: managedBy
properties: {}
}
```

In the new AVM modules its commented out and so its reverts to $null

```
resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
// managedBy: managedBy // removed due to immutable string, only used for managed resource groups
properties: {}
}
```
When testing, it will try and update the property but this is not allowed

```
"details": [
{
"code": "ResourceGroupManagedByMismatch",
"message": "The managed by property of the resource group cannot be changed from its current value ''."
}
]
```

### To reproduce

Deploy a resource group using the resource deployment of

```
resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
managedBy: managedBy
properties: {}
}
```

Update the code to comment out the managedBy property

```
resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
// managedBy: managedBy // removed due to immutable string, only used for managed resource groups
properties: {}
}
```

### Code snippet

```Bicep
resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
managedBy: managedBy
properties: {}
}

resource resourceGroup 'Microsoft.Resources/resourceGroups@2021-04-01' = {
location: location
name: name
tags: tags
// managedBy: managedBy // removed due to immutable string, only used for managed resource groups
properties: {}
}
```

### Relevant log output

```Shell
"details": [
{
"code": "ResourceGroupManagedByMismatch",
"message": "The managed by property of the resource group cannot be changed from its current value ''."
}
]
```