fix(server): http error parsing on endpoints without a default response (#12927)

mobile/openapi/lib/api/notifications_api.dart

@@ -48,10 +48,18 @@ class NotificationsApi {
   /// Parameters:
   ///
   /// * [SystemConfigSmtpDto] systemConfigSmtpDto (required):
-  Future<void> sendTestEmail(SystemConfigSmtpDto systemConfigSmtpDto,) async {
+  Future<TestEmailResponseDto?> sendTestEmail(SystemConfigSmtpDto systemConfigSmtpDto,) async {
     final response = await sendTestEmailWithHttpInfo(systemConfigSmtpDto,);
     if (response.statusCode >= HttpStatus.badRequest) {
       throw ApiException(response.statusCode, await _decodeBodyBytes(response));
     }
+    // When a remote server returns no body with a status of 204, we shall not decode it.
+    // At the time of writing this, `dart:convert` will throw an "Unexpected end of input"
+    // FormatException when trying to decode an empty string.
+    if (response.body.isNotEmpty && response.statusCode != HttpStatus.noContent) {
+      return await apiClient.deserializeAsync(await _decodeBodyBytes(response), 'TestEmailResponseDto',) as TestEmailResponseDto;
+    
+    }
+    return null;
   }
 }