fix: prevent empty file from overwriting existing bill attachments

Improve file attachment handling by:
- Adding fileSize === 0 check to prevent empty files from being processed
- Simplifying attachment logic by moving size validation to serializeAttachment function
- Ensuring null is returned for empty/invalid files to preserve existing attachments

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

Co-Authored-By: Claude <noreply@anthropic.com>

app/lib/actions/billActions.ts

@@ -86,7 +86,7 @@ const serializeAttachment = async (billAttachment: File | null) => {
         lastModified: fileLastModified,
     } = billAttachment;
 
-    if(!fileName || fileName === 'undefined') {
+    if(!fileName || fileName === 'undefined' || fileSize === 0) {
         return null;
     }
 
@@ -150,9 +150,7 @@ export const updateOrAddBill = withUser(async (user:AuthenticatedUser, locationI
     // update the bill in the mongodb
     const dbClient = await getDbClient();
     
-    const billAttachmentFile = formData.get('billAttachment') as File;
+    const billAttachment = await serializeAttachment(formData.get('billAttachment') as File);
-    const billAttachment = billAttachmentFile && billAttachmentFile.size > 0 ? 
-        await serializeAttachment(billAttachmentFile) : null;
 
     if(billId) {