fix(meetings): allow editing summary to empty string

Change validation to check if fields exist rather than if they're truthy, allowing users to clear summary content by editing to an empty string.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: Andres Tobon <andrest2455@gmail.com>

Author: andrest50

apps/lfx-one/src/server/controllers/past-meeting.controller.ts

@@ -284,7 +284,7 @@ export class PastMeetingController {
       const body = req.body as UpdatePastMeetingSummaryRequest;
 
       // Validate request body - at least one field must be provided
-      if (!body.edited_content && body.approved === undefined) {
+      if (!('edited_content' in body) && !('approved' in body)) {
         res.status(400).json({
           error: 'Bad Request',
           message: 'Either edited_content or approved must be provided',