//若clue1在数据库中的字段是数值型
string insSql = "INSETR INTO PUZZLE(CLUE1) VALUES (" + clue1 == "" ? "null" :clue1 + ")";
//若是字符型
string insSql="INSETR INTO PUZZLE(CLUE1) VALUES ("+clue1==""?"null":"'"+clue1+"')";

------解决方案--------------------
SQL code

declare @clue1 varchar(1)
insert into puzziw(clue1)
select case when @clue1='' then null else @clue1 end

------解决方案--------------------
InSert Into Puzzle(Clue1)
Select Case When ISNULL(clue1,'') = '' then Null Else clue1 End From 表