Don't default location to "Online" when scraper fails to extract venue

[Metrohan]

What to build / Ne yapılacak

EN: When a scraper cannot find a venue, it currently falls back to "Online" — which is factually wrong for in-person events where the location is simply unknown. The fallback should instead store null (or an empty string), and the frontend should display a human-readable placeholder such as "Konum belirtilmemiş" / "Venue not specified".

TR: Bir scraper konum bilgisini çekemediğinde şu an "Online" değerine düşüyor; bu, yüz yüze etkinlikler için yanlış veri üretiyor. Fallback değeri null (ya da boş string) olarak saklanmalı, frontend'de ise "Konum belirtilmemiş" gibi bir placeholder gösterilmeli.

Acceptance criteria

• Scraper functions no longer assign "Online" as a default when location extraction fails; they return None / "" instead.
• The public API serializes a missing location as null (not "Online").
• The frontend displays "Konum belirtilmemiş" (or equivalent) for events with a null/empty location.
• A unit test verifies that a scraper returning no location produces null in the persisted event, not "Online".

Blocked by

None — can start immediately. Pairs well with #17 (location normalization) but is independently mergeable.