allow empty plays again, for now (#55878)

changelogs/fragments/bad_empty_play.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - re allow empty plays for now, but add deprecation msg.

lib/ansible/playbook/__init__.py

@@ -69,12 +69,14 @@ class Playbook:
             raise AnsibleParserError("Could not read playbook (%s) due to encoding issues: %s" % (file_name, to_native(e)))
 
         # check for errors and restore the basedir in case this error is caught and handled
-        if not ds:
+        if ds is None:
             self._loader.set_basedir(cur_basedir)
             raise AnsibleParserError("Empty playbook, nothing to do", obj=ds)
         elif not isinstance(ds, list):
             self._loader.set_basedir(cur_basedir)
             raise AnsibleParserError("A playbook must be a list of plays, got a %s instead" % type(ds), obj=ds)
+        elif not ds:
+            display.deprecated("Empty plays will currently be skipped, in the future they will cause a syntax error", version='2.12')
 
         # Parse the playbook entries. For plays, we simply parse them
         # using the Play() object, and includes are parsed using the