store NULL when an element's discovery year is "Ancient"

The only elements that do not have a discovery year are those that
have been known for about as long as we have historical records.
It is highly unlikely that we will discover a new element that will
not have a definite year of discovery, so NULL can be used to refer
to elements so old nobody knows who discovered it first.

makedb.ml

@@ -96,6 +96,7 @@ let makedb db json = let open Sqlite3
 		in let x = (match colarr.(i) with
 		| "Symbol" -> symbs := (lcase x,None)::!symbs; lcase x
 		| "Name" -> names := (lcase x,None)::!names; lcase x
+		| "YearDiscovered" -> if x = "Ancient" then "" else x
 		| _ -> x
 		) in
 		binds stmt colarr.(i)