DATAJPA-438 - Removed custom ConversionService handling in JpaQueryExecution.

Spring Data Commons introduced a generic mechanism to post process the results of a repository method execution to potentially mitigate type differences between the result of the execution and the declared method return type. Thus we can remove the custom ConversionService usage implemented in JpaQueryExecution.

Related tickets: DATACMNS-483.
DATAJPA-517 - Defensive checks for CrudMethodMetadata in SimpleJpaRepository.

As CrudMethodMetadata is an optional dependency of SimpleJpaRepository we need to guard access to it with proper null checks.