Fix panic

[CascadingRadium]

need this check because i accidentally used the API without giving a k value it panicked because in

index.go in go-faiss you have

func (idx *faissIndex) Search(x []float32, k int64) (
	distances []float32, labels []int64, err error,
	) {
n := len(x) / idx.D()
distances = make([]float32, int64(n)*k)
labels = make([]int64, int64(n)*k)
if c := C.faiss_Index_search(
	idx.idx,
	C.idx_t(n),
	(*C.float)(&x[0]),
	C.idx_t(k),
	(*C.float)(&distances[0]),
	(*C.idx_t)(&labels[0]),

so if either k is 0 or query vector is nil/empty we get a panic because distances[0] is invalid operation