Properly wrap objects using json default encoder

Our custom encoder for the to_json filter was simply returning the
object if it was not a HostVars object, leading in some cases to a
TypeError when the data contained an undefined variable. This lead
to an odd error message being propagated up, so we now properly catch
this as an undefined variable error.

Fixes #15610

lib/ansible/plugins/filter/core.py

@@ -68,7 +68,7 @@ class AnsibleJSONEncoder(json.JSONEncoder):
         if isinstance(o, HostVars):
             return dict(o)
         else:
-            return o
+            return json.JSONEncoder.default(o)
 
 def to_yaml(a, *args, **kw):
     '''Make verbose, human readable yaml'''