Well, you could just filter them out:
pointList.Where(p => p != null).Max(p => p.X)
On the other hand, if you want nulls to be treated as though they were points having X-coordinate 0 (or similar), you could do:
pointList.Max(p => p == null ? 0 : p.X)
Do note that both techniques will throw if the sequence is empty. One workaround for this (if desirable) would be:
pointList.DefaultIfEmpty().Max(p => p == null ? 0 : p.X)