Fix Possible error when subscription is null in Get-AzRoleAssignment (#22776)

Author: NoriZC

src/Resources/Resources/RoleAssignments/GetAzureRoleAssignmentCommand.cs

@@ -240,7 +240,7 @@ public override void ExecuteCmdlet()
                     ResourceGroupName = ResourceGroupName,
                     ResourceName = ResourceName,
                     ResourceType = ResourceType,
-                    Subscription = string.IsNullOrEmpty(ResourceGroupName) ? null : DefaultProfile.DefaultContext.Subscription.Id
+                    Subscription = string.IsNullOrEmpty(ResourceGroupName) ? null : DefaultProfile.DefaultContext.Subscription?.Id?.ToString()
                 },
                 ExpandPrincipalGroups = ExpandPrincipalGroups.IsPresent,
                 IncludeClassicAdministrators = IncludeClassicAdministrators.IsPresent,