Properly handle user selection of `None` as vars_files (#31313)

* Properly handle user selection of `None` as vars_files

In a playbook, if a user has a playbook like:

```
- hosts: localhost
  connection: local
  vars_files:
  tasks:
  - ....
```

Then `vars_files` will be none, and cause a `TypeError` in vars-manager when it
tries to iterate over them. To avoid this, I changed the getter to either send
back the vars files from the user, or an empty list when the user passed
`None`.

* Only replace None with an empty list, not all falsey values

* Catch error when vars_files isn't iterable

* Move whole `for` loop into try/except and catch TypeError

* Line length

lib/ansible/playbook/play.py

@@ -277,6 +277,8 @@ class Play(Base, Taggable, Become):
         return self.vars.copy()
 
     def get_vars_files(self):
+        if self.vars_files is None:
+            return []
         return self.vars_files
 
     def get_handlers(self):

lib/ansible/vars/manager.py

@@ -346,7 +346,9 @@ class VariableManager:
         if play:
             all_vars = combine_vars(all_vars, play.get_vars())
 
-            for vars_file_item in play.get_vars_files():
+            vars_files = play.get_vars_files()
+            try:
+                for vars_file_item in vars_files:
                     # create a set of temporary vars here, which incorporate the extra
                     # and magic vars so we can properly template the vars_files entries
                     temp_vars = combine_vars(all_vars, self._extra_vars)
@@ -384,8 +386,8 @@ class VariableManager:
                                 raise AnsibleFileNotFound("vars file %s was not found" % vars_file_item)
                     except (UndefinedError, AnsibleUndefinedVariable):
                         if host is not None and self._fact_cache.get(host.name, dict()).get('module_setup') and task is not None:
-                        raise AnsibleUndefinedVariable("an undefined variable was found when attempting to template the vars_files item '%s'" % vars_file_item,
-                                                       obj=vars_file_item)
+                            raise AnsibleUndefinedVariable("an undefined variable was found when attempting to template the vars_files item '%s'"
+                                                           % vars_file_item, obj=vars_file_item)
                         else:
                             # we do not have a full context here, and the missing variable could be because of that
                             # so just show a warning and continue
@@ -393,6 +395,9 @@ class VariableManager:
                             continue
 
                     display.vvv("Read vars_file '%s'" % vars_file_item)
+            except TypeError:
+                raise AnsibleParserError("Error while reading vars files - please supply a list of file names. "
+                                         "Got '%s' of type %s" % (vars_files, type(vars_files)))
 
             # By default, we now merge in all vars from all roles in the play,
             # unless the user has disabled this via a config option