sd-device: make device_get_properties_{nulstr,strv}() take NULL for result value
    
    In most cases, it is not necessary to call them without retrieving
    result. But, most of other getter functions for sd-device can take NULL.
    Let's follow the way for consistency.
    
    (cherry picked from commit 793ab3e9dd733d743e1d3825a26ff65384ac3cbb)