Specify flexible generics nullness in spring-jdbc

## Overview

`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 tooling to deduce the nullability of `T` from the `ResultSetExtractor` handed into the method.
