WIP: add support for receiving single elements of lists

- resolved weird bug, that causes errors when nonterminal A (in the singleList-test) has no children.
- add new testcase for different structure/variants of some node to be sent/received
4 jobs for 21-allow-connection-endpoints-for-list-nonterminals in 10 minutes and 57 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Build
passed build #23875

00:01:53

 
  Test
passed test #23876

00:09:04

 
  Ragdoc Build
passed ragdoc_build #23877

00:00:36

 
  Ragdoc View
passed ragdoc_view #23878

00:03:02