Improve JdbcUserDetailsManager.userExists method

[Shenker93]

Hello everyone,

This proposal is about tiny enhancements applied to JdbcUserDetailsManager.userExists() method.
As we do not really use an information returned by select query
select username from users where username = ?,
and only resultset size is important, my idea is to replace it with an ordinary count query.
Some pros of this approach:

• It works faster on DB side and IMO the code is more straightforward in this case, you see what's the purpose just looking on sql
• No need to create unnecessary wrappers on Java side. Currently JdbcTemplate creates a list with a one single purpose - to let the code check its size - and then it goes directly to GC. Not used for iterating over it or any modification/read operations.
• Overloaded version of queryForList() method used there is already marked as @Deprecated as of Spring 5.3 release in favor of queryForList(String, Class, Object...). However that's a separate topic, some other JdbcUserDetailsManager methods also use it, e.g. findUsersInGroup(), that could be resolved in a separate PR to not mix the things up.

Also, a unit test was added in case if userExists() was invoked with a 'null' parameter. That use-case is not something that really-really should be used as most databases expect is [not] null query pattern instead (well, with rare exclusions for null-aware mode support), However as long as this method is designed in this fail-safe manner, let's create a test to be informed about any breaking changes.