A call to resolvePropertyValue can be very inefficient

Each time a property is retrieved, all system properties names are iterated.
This is because PropertyResolverUtil#resolvePropertyValue is calling GenericUtils.isEmpty(props) on a SyspropsMapWrapper.

This can be solved by using isNull(props) instead of isEmpty(props) as there's no need to check if the map is empty.
In the same call, there's also a call to System.getProperty which seems to be redundant with the SyspropsMapWrapper. I think it should be removed.