Copy extra stuff to SB on low diff

[dudehacker]

https://osu.ppy.sh/beatmapsets/982271#mania/2055771

Bug: copied notes to SB when there is no hitobject with keysound option unchecked.

[dudehacker]

After investigation:
Source contains Clap + tom.wav (in SB)
Target doesn't any note at that time

Current behavior: copies tom.wav to SB but loses the Clap
Expected behavior: nothing should copied with keysound option off.