(core) run survey for new users only when a survey doc is set

Summary:
stop providing a default document id DOC_ID_NEW_USER_INFO for
surveying, and don't show survey if a document id is not available.

Test Plan: existing tests pass; grist-core checked

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3225

app/server/lib/sendAppPage.ts

@@ -46,6 +46,7 @@ export function makeGristConfig(homeUrl: string|null, extra: Partial<GristLoadCo
     maxUploadSizeAttachment: (Number(process.env.GRIST_MAX_UPLOAD_ATTACHMENT_MB) * 1024 * 1024) || undefined,
     timestampMs: Date.now(),
     enableWidgetRepository: Boolean(process.env.GRIST_WIDGET_LIST_URL),
+    survey: Boolean(process.env.DOC_ID_NEW_USER_INFO),
     ...extra,
   };
 }