Windows: Small fix to Add-Warning and Add-DeprecationWarning (#23520)

Simplified the logic, and fix the issue of having a nested list as
entry.

Also indicate that we deliberately want to retain $null values
(and i.e. do not want them to be turned into a boolean).

test/integration/targets/win_find/tasks/main.yml

@@ -131,7 +131,7 @@
   win_find:
     patterns: a
   register: actual
-  failed_when: "actual.msg != 'Missing required argument: paths'"
+  failed_when: "actual.msg != 'Get-AnsibleParam: Missing required argument: paths'"
 
 - name: expect failure when setting paths to a file
   win_find:

test/integration/targets/win_reg_stat/tasks/main.yml

@@ -29,7 +29,7 @@
   win_reg_stat:
     name: a
   register: actual
-  failed_when: "actual.msg != 'Missing required argument: path'"
+  failed_when: "actual.msg != 'Get-AnsibleParam: Missing required argument: path'"
 
 - name: expect failure when passing in an invalid hive
   win_reg_stat: