Summary:
UpgradeYsql can be invoked by the user via the yb-admin `upgrade_ysql` command. If the user calls this on a universe without ysql, then we should early exist the function.

Fixes #27167
Jira: DB-16653

Test Plan: TEST_F(YcqlOnlyUpgradeTest, TestUpgrade) {

Reviewers: telgersma

Reviewed By: telgersma

Subscribers: ybase

Differential Revision: https://phorge.dev.yugabyte.com/D45096