[Az.Storage] Set-AzCurrentStorageAccount can't set context with Oauth credential

## Description

Set-AzCurrentStorageAccount can't set context with Oauth credential

## Steps to reproduce

```powershell

PS C:\Users\weiwei> $oauthCtx = New-AzStorageContext -StorageAccountName $accountName

PS C:\Users\weiwei> Set-AzCurrentStorageAccount -Context $oauthCtx
Set-AzCurrentStorageAccount : Object reference not set to an instance of an object.
At line:1 char:1
+ Set-AzCurrentStorageAccount -Context $oauthCtx
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Set-AzCurrentStorageAccount], NullReferenceException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Management.Storage.SetAzureRmCurrentStorageAccount
```

## Environment data

```
Not related
```

## Module versions

```powershell
Az.Storage 3.2.1
```

## Debug output

```
PS C:\> Set-AzCurrentStorageAccount -Context $oauthCtx -debug
DEBUG: 5:48:31 PM - SetAzureRmCurrentStorageAccount begin processing with ParameterSet 'UsingStorageContext'.
DEBUG: 5:48:32 PM - using account id '[hidden]'...
Set-AzCurrentStorageAccount : Object reference not set to an instance of an object.
At line:2 char:1
+ Set-AzCurrentStorageAccount -Context $oauthCtx -debug
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Set-AzCurrentStorageAccount], NullReferenceException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Management.Storage.SetAzureRmCurrentStorageAccount

DEBUG: 5:48:33 PM - SetAzureRmCurrentStorageAccount end processing.
```

## Error output

```
Set-AzCurrentStorageAccount : Object reference not set to an instance of an object.
At line:1 char:1
+ Set-AzCurrentStorageAccount -Context $oauthCtx
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Set-AzCurrentStorageAccount], NullReferenceException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Management.Storage.SetAzureRmCurrentStorageAccount
```