[psetv2] Hotfix: optional rng func arg

Author: altafan

confidential/confidential.go

@@ -434,6 +434,9 @@ func CreateBlindValueProof(
 	ctx, _ := secp256k1.ContextCreate(secp256k1.ContextBoth)
 	defer secp256k1.ContextDestroy(ctx)
 
+	if rng == nil {
+		rng = generateRandomNumber
+	}
 	r, err := rng()
 	if err != nil {
 		return nil, err