expression: speed up Column.VecEvalReal by using MergeNulls

[Tjianke]

General Question

I found a todo item('speed up the way to manipulate null-bitmaps') in expression/column.go, so I was thinking maybe using Column.MergeNulls() would be better than setting null on every single bit in the bitmap.

In original code, we have to set null for every null element in nullbitmap, using SetNull() method.

        n := input.NumRows()
	src := input.Column(col.Index)
	if col.GetType().Tp == mysql.TypeFloat {
		result.ResizeFloat64(n, false)
		f32s := src.Float32s()
		f64s := result.Float64s()
		for i := range f32s {
			// TODO(zhangyuanjia): speed up the way to manipulate null-bitmaps.
			if src.IsNull(i) {
				result.SetNull(i, true)
			} else {
				f64s[i] = float64(f32s[i])
			}
		}
		return nil
	}

Theoretically it would be more efficient if we replace Column.SetNull() with Column.MergeNulls(), which utilizes bit operation underneath.

Proposed method:

        n := input.NumRows()
	src := input.Column(col.Index)
	if col.GetType().Tp == mysql.TypeFloat {
		result.ResizeFloat64(n, false)
		f32s := src.Float32s()
		f64s := result.Float64s()
                result.MergeNulls(src)
		for i := range f32s {
			if src.IsNull(i) {
				continue
			} 
           		f64s[i] = float64(f32s[i])		
		}
		return nil
	}