The first one, let's call it @null:

whileprintingrecords;
stringvar names := "";

Place it in your GH section, it will return you null.

The second one, let's say @comma_field:

whileprintingrecords;
stringvar names;
names := names & "---- " & {table.field};

Place it in your Detailes section which you can suppress; the formula will return you next

and the 3rd formula, let's call it @final :

whileprintingrecords;
stringvar names;
Mid(names, 3);

Place it in your GF or report section.

As per my understanding , you want to suppress the Checktime rows whenever attendance value is Null. If yes, then