Remove redundant null check in HikariJdbcConnectionDetailsBeanPostProcessor

[dmitrysulman]

This PR removes an unnecessary null check for driverClassName in the HikariJdbcConnectionDetailsBeanPostProcessor, as JdbcConnectionDetails#getDriverClassName is guaranteed to return a non-null value.

[philwebb]

Thanks for the PR. Whilst I agree that JdbcConnectionDetails.getDriverClassName() should not return null, there's still the possibility of that happening if a user implements the interface and overrides the default method. It looks like we added the null check for #44997, and whilst the test in that issue no longer causes a problem, I'm not sure we should rush to remove the defensive code we currently have.

Thanks anyway for the suggestion.