randomizer: do not pass NULL to memcpy()

When switching playback order from NORMAL to RANDOM, the current playlist items are added to the randomizer.

However, if the playlist is empty, the items vector is empty, and its `.data field is NULL. This causes the following error in ASAN:

../../src/playlist/randomizer.c:426:10: runtime error: null pointer passed as argument 2, which is declared to never be null

If the playlist size is 0, then do not call randomizer_Add() at all.