fix(api): convert empty strings to null for date/optional DB columns

Course and event creation Server Actions were failing with 'Something went
wrong' because empty form strings ('') were being inserted into date/uuid
columns which reject empty strings. Now converts '' to null for all
optional fields (dates, descriptions, IDs, contact info).

Dockerfile

@@ -5,7 +5,7 @@ WORKDIR /app
 # --- Install + Build in one stage ---
 FROM base AS builder
 # CACHE_BUST: change this value to force a full rebuild (busts Docker layer cache)
-ARG CACHE_BUST=10
+ARG CACHE_BUST=11
 RUN echo "Cache bust: ${CACHE_BUST}"
 COPY . .
 RUN pnpm install --no-frozen-lockfile