caller to preprocess must be None aware (#33653)

also removed unused copy of method

fixes #33650

lib/ansible/playbook/play.py

@@ -195,6 +195,7 @@ class Play(Base, Taggable, Become):
     def _load_vars_prompt(self, attr, ds):
         new_ds = preprocess_vars(ds)
         vars_prompts = []
+        if new_ds is not None:
             for prompt_data in new_ds:
                 if 'name' not in prompt_data:
                     display.deprecated("Using the 'short form' for vars_prompt has been deprecated", version="2.7")

lib/ansible/vars/manager.py

@@ -151,26 +151,6 @@ class VariableManager:
             raise AnsibleAssertionError("the type of 'value' for options_vars should be a dict, but is a %s" % type(value))
         self._options_vars = value.copy()
 
-    def _preprocess_vars(self, a):
-        '''
-        Ensures that vars contained in the parameter passed in are
-        returned as a list of dictionaries, to ensure for instance
-        that vars loaded from a file conform to an expected state.
-        '''
-
-        if a is None:
-            return None
-        elif not isinstance(a, list):
-            data = [a]
-        else:
-            data = a
-
-        for item in data:
-            if not isinstance(item, MutableMapping):
-                raise AnsibleError("variable files must contain either a dictionary of variables, or a list of dictionaries. Got: %s (%s)" % (a, type(a)))
-
-        return data
-
     def get_vars(self, play=None, host=None, task=None, include_hostvars=True, include_delegate_to=True, use_cache=True):
         '''
         Returns the variables, with optional "context" given via the parameters