Get-AzDelegation returns an error when null

```
CommandType Name Version Source
----------- ---- ------- ------
Cmdlet Get-AzDelegation 3.3.0 Az.Network
```

When I run the command as below an error is returned (the delegation does not exist) :-
```
Get-AzDelegation -Name nmeFunction -Subnet $subnet
Get-AzDelegation : Sequence contains no matching element
At line:1 char:1
+ Get-AzDelegation -Name nmeFunction -Subnet $subnet
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Get-AzDelegation], InvalidOperationException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Network.GetAzureDelegation
```
Would it be better if ```$null``` is returned instead when an object doesn't exist?

Start by reproducing the Get-AzDelegation command from the issue with a nonexistent delegation and inspect the Az.Network implementation that handles the subnet lookup. Done means the command returns $null when no matching delegation exists instead of raising “Sequence contains no matching element”; add or update coverage for this case if the project’s existing tests expose the command.

Written by the indexing model from the issue text.