fix(ast_utils): use None sentinel for walk_type_tree seen parameter

[SoundMatt]

walk_type_tree declares seen={} as a default argument. Python evaluates default argument values once at function-definition time, so every call that omits seen shares the same dict object. After the first call the dict already contains all visited type names, meaning every subsequent call skips every node and does nothing.

Fix with the standard sentinel pattern: seen=None, initialised to {} inside the function body.

[gunnar-mb]

"After the first call the dict retains every visited type name, so a second call would immediately short-circuit on
every node and do nothing."

OK, fo recursive call to itself, it is the intended behavior that previously "seen" items should be skipped (but seen is explicitly passed to ensure this).

We also have not come across this in other cases because most of the IFEX programs are individually executed and only one instance of walk_type_tree is usually invoked. But this bug would be triggered if two different walk_type_tree sessions are started in the same program.

It looks correct but let's make sure there is a unit-test covering it.