JdbcClient ResultQuerySpec - Provide a optionalValue() method

[vcruzmj]

The current singleValue() on the interface ResultQuerySpec expects 1 mandatory result, making confuse to get 0 or 1 results, wich is very common to have a query that returns 1 or 0 results, like this

SELECT somecolumn
FROM sometable
WHERE :somefilter
LIMIT 1

If that filter is not meet, the result is empty, and singleValue() is not usefull

Current implementation:

default Object singleValue() {
    return DataAccessUtils.requiredSingleResult(singleColumn());
}

The current implementation can be enhanced by just adding this extra method (using the existing DataAccessUtils method optionalResult.

default Object singleValue() {
    return DataAccessUtils.requiredSingleResult(singleColumn());
}

default Optional<Object> optionalValue() {
    return DataAccessUtils.optionalResult(singleColumn());
}

This will also help maintain coherence with the single() and optional() methods already provided on MappedQuerySpec.

[jhoeller]

Good point, this goes together nicely with #33300 where we are enforcing non-null values contained in the result now.