fix(ifex_to_json_schema): use None sentinels for mutable default args

[SoundMatt]

collect_type_info(t, collection={}, seen={}) uses mutable dicts as default arguments. Python creates these once at function-definition time and reuses the same objects on every call that omits those arguments.

After the first call:

• seen retains all visited type names, so any subsequent call skips every node and collects nothing.
• collection accumulates entries from previous calls, polluting results.

Fix with the standard None-sentinel pattern.

[gunnar-mb]

Same as in other places but the described problem is never triggered because the function (the whole module) is only run once. Maybe if the function was shown to be internal to the module, this would be evident to Claude.

Anyhow, I guess the pattern is considered bad practice, so it should be avoided.

[gunnar-mb]

It looks good anyhow - will either merge it, or combine it with the same style of change in other modules.