Import-AzBlueprintWithArtifact fails with "Object reference not set to an instance of an object" if executed for a tenant with no subscriptions (or if no subscription is available in the context)

## Description

I'm trying to create a blueprint in a fresh tenant with no subscriptions associated with it yet. It works fine in the portal but Az.Blueprint cmdlets fail (not just Import-AzBlueprintWithArtifact but also New-AzBlueprint). Authentication is performed with a service principal though I've tried with a regular user with the same result. The same cmdlets work fine for another tenant with a subscription associated to it.

## Steps to reproduce

```powershell

1. Create or use a tenant with no subscriptions associated to it. Another option is to use a service principal or a user having no role assignments to existing subscriptions within the tenant (Get-AzContext will show no subscription in the output - see below).
2. Authenticate to Azure (e.g. with Login-AzAccount).
3. Run Import-AzBlueprintWithArtifact or New-AzBlueprint (even fake parameter values are sufficient for this repro as the cmdlets fail before verifying that parameters are valid).

```

## Environment data

```
PS C:\Current\Repro> $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

PS C:\Current\Repro> Get-AzContext

Name Account SubscriptionName Environment TenantId
---- ------- ---------------- ----------- --------
6403b9e7-710d-43cb-a9e9-cb8d446174fa ... df05ee65-2d30-4670-8f08-b118ec9... AzureCloud 6403b9e7-710d-43cb-a9e9-cb8d44...

```

## Module versions

```powershell
PS C:\Current\Repro> get-module -listavailable -Name Az.Accounts

Directory: C:\Users\MGaufman\Documents\WindowsPowerShell\Modules

ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 1.7.2 Az.Accounts {Disable-AzDataCollection, Disable-AzContextAutosave, Enable-AzDataCollection, Enable-AzContextAutosave...}
Script 1.7.1 Az.Accounts {Disable-AzDataCollection, Disable-AzContextAutosave, Enable-AzDataCollection, Enable-AzContextAutosave...}
Script 1.6.4 Az.Accounts {Disable-AzDataCollection, Disable-AzContextAutosave, Enable-AzDataCollection, Enable-AzContextAutosave...}
Script 1.3.0 Az.Accounts {Disable-AzDataCollection, Disable-AzContextAutosave, Enable-AzDataCollection, Enable-AzContextAutosave...}

PS C:\Current\Repro> get-module -listavailable -Name Az.Blueprint

Directory: C:\Users\MGaufman\Documents\WindowsPowerShell\Modules

ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 0.2.10 Az.Blueprint {Get-AzBlueprint, Get-AzBlueprintAssignment, New-AzBlueprintAssignment, Remove-AzBlueprintAssignment...}

PS C:\Current\Repro> get-module

ModuleType Version Name ExportedCommands
---------- ------- ---- ----------------
Script 1.7.2 Az.Accounts {Add-AzEnvironment, Clear-AzContext, Clear-AzDefault, Connect-AzAccount...}
Script 0.2.10 Az.Blueprint {Export-AzBlueprintWithArtifact, Get-AzBlueprint, Get-AzBlueprintArtifact, Get-AzBlueprintAssignment...}
Manifest 3.1.0.0 Microsoft.PowerShell.Management {Add-Computer, Add-Content, Checkpoint-Computer, Clear-Content...}
Manifest 3.0.0.0 Microsoft.PowerShell.Security {ConvertFrom-SecureString, ConvertTo-SecureString, Get-Acl, Get-AuthenticodeSignature...}
Manifest 3.1.0.0 Microsoft.PowerShell.Utility {Add-Member, Add-Type, Clear-Variable, Compare-Object...}
Manifest 3.0.0.0 Microsoft.WSMan.Management {Connect-WSMan, Disable-WSManCredSSP, Disconnect-WSMan, Enable-WSManCredSSP...}
Script 2.0.0 PSReadline {Get-PSReadLineKeyHandler, Get-PSReadLineOption, Remove-PSReadLineKeyHandler, Set-PSReadLineKeyHandler...}

```

## Debug output

```
PS C:\current\Repro> Import-AzBlueprintWithArtifact -Name "bp" -ManagementGroupId "6403b9e7-710d-43cb-a9e9-cb8d446174fa" -InputPath "bpdef"
DEBUG: 15:20:26 - ImportAzureRmBlueprint begin processing with ParameterSet 'ImportBlueprint'.
DEBUG: 15:20:26 - using account id 'df05ee65-2d30-4670-8f08-b118ec938b04'...
Import-AzBlueprintWithArtifact : Object reference not set to an instance of an object.
At line:1 char:1
+ Import-AzBlueprintWithArtifact -Name "bp" -ManagementGroupId "6403b9e ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Import-AzBlueprintWithArtifact], NullReferenceException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Blueprint.Cmdlets.ImportAzureRmBlueprint

DEBUG: 15:20:27 - ImportAzureRmBlueprint end processing.
```

## Error output

```
PS C:\current\Repro> Resolve-AzError
DEBUG: 15:20:52 - ResolveError begin processing with ParameterSet 'AnyErrorParameterSet'.
DEBUG: 15:20:52 - using account id 'df05ee65-2d30-4670-8f08-b118ec938b04'...
WARNING: Breaking changes in the cmdlet 'Resolve-AzError' :
WARNING: - The `Resolve-Error` alias will be removed in a future release. Please change any scripts that use this alias to use `Resolve-AzError` instead.

WARNING: NOTE : Go to https://aka.ms/azps-changewarnings for steps to suppress this breaking change warning, and other information on breaking changes in Azure PowerShell.

HistoryId: 3

Message : Object reference not set to an instance of an object.
StackTrace : at Microsoft.Azure.Commands.Blueprint.Cmdlets.ImportAzureRmBlueprint.ExecuteCmdlet()
at Microsoft.WindowsAzure.Commands.Utilities.Common.AzurePSCmdlet.ProcessRecord()
Exception : System.NullReferenceException
InvocationInfo : {Import-AzBlueprintWithArtifact}
Line : Import-AzBlueprintWithArtifact -Name "bp" -ManagementGroupId "6403b9e7-710d-43cb-a9e9-cb8d446174fa" -InputPath "bpdef"
Position : At line:1 char:1
+ Import-AzBlueprintWithArtifact -Name "bp" -ManagementGroupId "6403b9e ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
HistoryId : 3

DEBUG: 15:20:52 - ResolveError end processing.
```