Makes final function and initial condition to be optional while
creating UDA. No final function means UDA returns final state
and default initial condition is `null`.
Both items were optional in cql's grammar but they were treated as required in code.
Additionally I've added check if state function returns state.
Fixes#10324Closes#10331
* github.com:scylladb/scylla:
CQL3: check sfunc return type in UDA
cql-pytest: UDA no final_func/initcond tests
cql3: allow no final_func and no initcond in UDA
2683b54402