Improve nullability declarations for return types in `[NamedParameter]JdbcOperations`

`JdbcOperations.query(String, ResultSetExtractor<T>)` is currently declared as `@Nullable`. Tweaking the declaration to:

```java
<T extends @Nullable Object> T query(String sql, ResultSetExtractor<T> rse) throws DataAccessException;
```

would allow the tooling to deduce the nullability of `T` from the `RSE` handed into the method.