Fix roaring test: TestBitmap_Quick_Array1
Fix roaring test: TestBitmap_Quick_Array1

When the random number generator was coming up 0,
then `a = []uint64{}`. This caused the bitmap `bm` to be empty.
In this case, `bm.Slice()` is a nil slice, while the expected
slice is an empty slice (i.e. they are not considered equal):
got: ([]uint64) <nil>
exp: ([]uint64) {}

linhvo/pilosa,alanbernstein/pilosa,raskle/pilosa,alanbernstein/pilosa,linhvo/pilosa,linhvo/pilosa,raskle/pilosa,travisturner/pilosa,raskle/pilosa,alanbernstein/pilosa