Ensure `when` warning is checked before expanding (#25092)

The `when` condition templating warning should only happen
if the condition itself contains templating, not if variables
in the condition are themselves composed through templating

Before

```
vars:
  x: hello
  y: "{{ x }}"

tasks:
- debug: msg=hello
  when: y
```

would fire a warning because `y` would get expanded to `{{ x }}`.
This checks whether a warning is required prior to expansion.

lib/ansible/playbook/conditional.py

@@ -127,6 +127,11 @@ class Conditional:
         if conditional is None or conditional == '':
             return True
 
+        if templar.is_template(conditional):
+            display.warning('when statements should not include jinja2 '
+                            'templating delimiters such as {{ }} or {%% %%}. '
+                            'Found: %s' % conditional)
+
         # pull the "bare" var out, which allows for nested conditionals
         # and things like:
         # - assert:
@@ -137,11 +142,6 @@ class Conditional:
         if conditional in all_vars and VALID_VAR_REGEX.match(conditional):
             conditional = all_vars[conditional]
 
-        if templar.is_template(conditional):
-            display.warning('when statements should not include jinja2 '
-                            'templating delimiters such as {{ }} or {%% %%}. '
-                            'Found: %s' % conditional)
-
         # make sure the templar is using the variables specified with this method
         templar.set_available_variables(variables=all_vars)