Default include_role results to empty list in linear strategy plugin

Fixes #18544.

When a loop is over an empty list, the result is set to

    {'skipped_reason': u'No items in the list', 'skipped': True, 'changed': False}

which means that accessing `hr._result['results']` throws a `KeyError`.

lib/ansible/plugins/strategy/linear.py

@@ -287,7 +287,7 @@ class StrategyModule(StrategyBase):
                             loop_var = 'item'
                             if hr._task.loop_control:
                                 loop_var = hr._task.loop_control.loop_var or 'item'
-                            include_results = hr._result['results']
+                            include_results = hr._result.get('results', [])
                         else:
                             include_results = [ hr._result ]