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).

packages/features/event-management/src/server/api.ts

@@ -47,12 +47,12 @@ export function createEventManagementApi(client: SupabaseClient<Database>) {
 
     async createEvent(input: CreateEventInput) {
       const { data, error } = await client.from('events').insert({
-        account_id: input.accountId, name: input.name, description: input.description,
-        event_date: input.eventDate, event_time: input.eventTime, end_date: input.endDate,
-        location: input.location, capacity: input.capacity, min_age: input.minAge,
-        max_age: input.maxAge, fee: input.fee, status: input.status,
-        registration_deadline: input.registrationDeadline,
-        contact_name: input.contactName, contact_email: input.contactEmail, contact_phone: input.contactPhone,
+        account_id: input.accountId, name: input.name, description: input.description || null,
+        event_date: input.eventDate || null, event_time: input.eventTime || null, end_date: input.endDate || null,
+        location: input.location || null, capacity: input.capacity, min_age: input.minAge ?? null,
+        max_age: input.maxAge ?? null, fee: input.fee, status: input.status,
+        registration_deadline: input.registrationDeadline || null,
+        contact_name: input.contactName || null, contact_email: input.contactEmail || null, contact_phone: input.contactPhone || null,
       }).select().single();
       if (error) throw error;
       return data;