fix: handle missing image_url in kalshi event normalizer

[realfishsam]

Fixes #442

[realfishsam]

PR Review: VERIFIED

What This Does

Fixes the Kalshi event normalizer to handle a missing image_url field by coercing null to undefined via the nullish coalescing operator. Previously, a null value from the API would propagate as null into UnifiedEvent.image, which is typed as string | undefined (not string | null).

Blast Radius

• core/src/exchanges/kalshi/normalizer.ts -- single line change in normalizeEvent

Findings

No blocking findings. The change is correct: raw.image_url ?? undefined converts null to undefined while passing through valid strings unchanged. This aligns with the image?: string type on UnifiedEvent.

PMXT Pipeline Check

• Field propagation: N/A (no new fields)
• OpenAPI sync: N/A
• Type safety: OK (fixes a null-vs-undefined type mismatch)

Semver Impact

patch -- bug fix for null handling.

Risk

None. Single-line, narrowly scoped fix.