Ensure _allEnvelopes is never null after assignment

Modified the assignment of `_allEnvelopes` to use the result of
`EnvelopeService.GetAsync()` or an empty list (`[]`) if the
result is null. This change prevents potential null reference
issues when `_allEnvelopes` is used later in the code.

EnvelopeGenerator.ReceiverUI/Pages/EnvelopeSenderPage.razor

@@ -352,7 +352,7 @@
 
         try
         {
-            _allEnvelopes = await EnvelopeService.GetAsync();
+            _allEnvelopes = await EnvelopeService.GetAsync() ?? [];
             
             // Split into active and completed based on status
             var envelopes = _allEnvelopes.ToList();