[MLv2] [FE] Migrate _serializeForUrl

[ranquild]

Blocked by #37659

metabase/frontend/src/metabase-lib/Question.ts

Line 992 in ba93751

const query = clean ? this.legacyQuery().clean() : this.legacyQuery();

• Lib.dropStageIfEmpty(query, -1) is a replacement for clean
• Lib.toLegacyQuery is a replacement for datasetQuery; i.e. it would serialize the query.

Using of toLegacyQuery caused some inner checks in initializeQB to work differently, that's why this issue is blocked by some changes in somewhat unrelated MLv2 function.