Issue #3151980 by anmolgoyal74, huzooka, benjifisher, Wim Leers, quietone:...

Issue #3151980 by anmolgoyal74, huzooka, benjifisher, Wim Leers, quietone: System mail settings migration (d7_system_mail) assumes that the mail_system variable is always available

core/modules/system/migrations/d7_system_mail.yml

@@ -5,7 +5,7 @@ migration_tags:
   - Configuration
 source:
   plugin: variable
-  variables:
+  variables_no_row_if_missing:
     - mail_system
   source_module: system
 process: