Fix notification Group property being overwritten in unpackaged apps (#3836)

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## Fixes #3835
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For unpackaged apps, we were accidently overwriting the Group property on toasts if developers were setting that. Thus, scenarios like #3835 where a progress bar toast is using both group and tag, updating the progress bar didn't work

## PR Type
What kind of change does this PR introduce?
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- Bugfix
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<!-- - Code style update (formatting) -->
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<!-- - Sample app changes -->
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## What is the current behavior?
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Group property gets overwritten

## What is the new behavior?
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Group property only gets written if developers didn't specify it


## PR Checklist

Verified via...
 - [x] Sample app modified with both using group and not using group for progress bar (and for removing toasts)
 - [ ] Final NuGet package in separate sample app (pending a build) 

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- [x] Contains **NO** breaking changes

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## Other information

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.Notifications/Toasts/Compat/ToastNotifierCompat.cs

@@ -143,10 +143,10 @@ private void PreprocessToast(ToastNotification notification)
             // For apps that don't have identity...
             if (!DesktopBridgeHelpers.HasIdentity())
             {
-                // If tag is specified
-                if (!string.IsNullOrEmpty(notification.Tag))
+                // If tag is specified and group isn't specified
+                if (!string.IsNullOrEmpty(notification.Tag) && string.IsNullOrEmpty(notification.Group))
                 {
-                    // If group isn't specified, we have to add a group since otherwise can't remove without a group
+                    // We have to add a group since otherwise can't remove without a group
                     notification.Group = ToastNotificationManagerCompat.DEFAULT_GROUP;
                 }
             }
@@ -157,10 +157,10 @@ private void PreprocessScheduledToast(ScheduledToastNotification notification)
             // For apps that don't have identity...
             if (!DesktopBridgeHelpers.HasIdentity())
             {
-                // If tag is specified
-                if (!string.IsNullOrEmpty(notification.Tag))
+                // If tag is specified and group isn't specified
+                if (!string.IsNullOrEmpty(notification.Tag) && string.IsNullOrEmpty(notification.Group))
                 {
-                    // If group isn't specified, we have to add a group since otherwise can't remove without a group
+                    // We have to add a group since otherwise can't remove without a group
                     notification.Group = ToastNotificationManagerCompat.DEFAULT_GROUP;
                 }
             }