fix(analytics): omit empty workspace_id on workspace-less file downloads

Address Greptile P1: the generic key-based /api/files/download route and the
no-workspace markdown-export path emitted file_downloaded with workspace_id:''
creating a phantom '' bucket in PostHog. Make workspace_id optional on the event
and omit it when there is no workspace (workspace-scoped routes still pass it).

Author: waleedlatif1

apps/sim/app/api/files/download/route.ts

@@ -98,7 +98,6 @@ export const POST = withRouteHandler(async (request: NextRequest) => {
       request,
     })
     captureServerEvent(userId, 'file_downloaded', {
-      workspace_id: '',
       is_bulk: false,
       file_count: 1,
     })

apps/sim/app/api/files/export/[id]/route.ts

@@ -97,7 +97,7 @@ export const GET = withRouteHandler(
         userId,
         'file_downloaded',
         {
-          workspace_id: record.workspaceId ?? '',
+          ...(record.workspaceId ? { workspace_id: record.workspaceId } : {}),
           is_bulk: assetCount > 0,
           file_count: 1 + assetCount,
         },

apps/sim/lib/posthog/events.ts

@@ -447,7 +447,7 @@ export interface PostHogEventMap {
   }
 
   file_downloaded: {
-    workspace_id: string
+    workspace_id?: string
     is_bulk: boolean
     file_count: number
   }