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

lib/ansible/executor/task_executor.py

@@ -167,14 +167,21 @@ 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_ 
-                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:
-                    if 'has no attribute' in str(e):
-                        loop_terms = []
-                        self._display.deprecated("Skipping task due to undefined attribute, in the future this will be a fatal error.")
-                    else:
-                        raise
+                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:
+                        if 'has no attribute' in str(e):
+                            loop_terms = []
+                            self._display.deprecated("Skipping task due to undefined attribute, in the future this will be a fatal error.")
+                        else:
+                            raise
                 items = self._shared_loader_obj.lookup_loader.get(self._task.loop, loader=self._loader, templar=templar).run(terms=loop_terms, variables=vars_copy)
             else:
                 raise AnsibleError("Unexpected failure in finding the lookup named '%s' in the available lookup plugins" % self._task.loop)