Do not assert session name on idempotent eos banner (#23402)

* Check for proper response key on eos_banner map_config_to_obj

If we run the task with 'login' banner, the 'show banner' command
will return a dict containing key 'loginBanner'.
However for motd, it will just return 'motd'.
Yay naming consistency!

* Do not assert session exists on eos_banner response not changing device

test/integration/targets/eos_banner/tests/eapi/basic-motd.yaml

@@ -47,7 +47,7 @@
       - "result.changed == false"
       - "result.commands | length == 0"
       # Ensure sessions contains epoc. Will fail after 18th May 2033
-      - "result.session_name contains 'ansible_1'"
+      - "result.session_name is not defined"
 
 # FIXME add in tests for everything defined in docs
 # FIXME Test state:absent + test: