fix: coerce null eventType to empty string in _parse_card_note (#350)

[SAY-5]

Closes #350.

_parse_card_note reads event_dict.get("eventType", ""), but dict.get(k, default) only returns the default when the key is absent. The Trade Republic timeline includes "Aktien erhalten" (shares-received) entries where the key is present with value null, so eventTypeStr becomes None and the next line's eventTypeStr.startswith("card_") nil-derefs, breaking pytr export_transactions CSV export end-to-end.

Replaces the default with event_dict.get("eventType") or "" so both missing and null collapse to an empty string. Regression test test_parse_card_note_returns_none_when_event_type_is_null covers the null case (the bug repro), the existing absent case, and the existing card_-prefix happy path.

[RealCLanger]

Good catch. Could you maybe add a full test for an "Aktien erhalten" event?

[SAY-5]

Sure. I don't have a real 'Aktien erhalten' payload handy. From #350's traceback it looks like the original event had eventType=null and the title 'AI & Big Data USD (Acc) Aktien erhalten'. Do you have a sanitized fixture I can drop into tests/aktien_erhalten*.json, or is the existing aktiensplit_no_eventType.json (which already carries the 'Du hast Aktien aus einer Kapitalmaßnahme erhalten' header) close enough to repurpose?