When playing Riven, does the Sand Soldier from Emperor's Dais enter Ready?

Yes — the Sand Soldier enters ready under Riven, The Exile.

Tokens count as "playing a unit" when an effect uses the word "play" (Dais says "play a 2 [S] Sand Soldier unit token here"), so Riven's replacement effect applies to it. Dais triggers on conquer — meaning control of that battlefield is already yours when the trigger resolves — so the token is played to a battlefield you control and enters ready instead of exhausted (CR 369.3, FAQ #9982).

Only the "played to" location matters; if some other effect later moved it, it would still have entered ready.