For with_first_found loops, set undefined variables to fall through rather than failing the task

lib/ansible/executor/task_executor.py

@@ -167,6 +167,13 @@ class TaskExecutor:
         if self._task.loop:
             if self._task.loop in self._shared_loader_obj.lookup_loader:
                 #TODO: remove convert_bare true and deprecate this in with_ 
+                if self._task.loop == 'first_found':
+                    # first_found loops are special.  If the item is undefined
+                    # then we want to fall through to the next value rather
+                    # than failing.
+                    loop_terms = listify_lookup_plugin_terms(terms=self._task.loop_args, templar=templar, loader=self._loader, fail_on_undefined=False, convert_bare=True)
+                    loop_terms = [t for t in loop_terms if not templar._contains_vars(t)]
+                else:
                     try:
                         loop_terms = listify_lookup_plugin_terms(terms=self._task.loop_args, templar=templar, loader=self._loader, fail_on_undefined=True, convert_bare=True)
                     except AnsibleUndefinedVariable as e: