Fix ambiguity in the JpaExecutor (#2640)

* Fix ambiguity in the `JpaExecutor`

Since we can't mutate a provided `ParameterSourceFactory` with the
provided `jpaParameters`, we have to reject such a combined
configuration in favor of the advice to configure params on the
provided `ParameterSourceFactory`

* Throw `IllegalStateException` when `ParameterSourceFactory` and
`jpaParameters` are configured together on the `JpaExecutor`
* Some refactoring for `JpaExecutor` to have a consistent code style
* Fix tests according new logic
* Document such an ambiguity configuration to avoid confusion

**Cherry-pick to 5.0.x**

* * Use `assertThatThrownBy()` in the `JpaExecutorTests`
* Fix "No BeanFactory" warning in the `JpaExecutorTests`

* * Resolve potential NPE and add `@NonNullApi` with `@Nullable` on the `poll()`

Author: artembilan