Coalesce with Timestamp,Nulls,and Concatenation

can someone please give the correct syntax for a query that has a column
timestamp that allows nulls and
needs to be converted to a certain format and also needs to be concatenated
with commas

example:

select to_char(timestamp1,'YYYYMMDD
HH24:MI')||','||employee||','||to_char(timestamp2,'YYYYMMDD HH24:MI') from
employee;

How do I put the coalesce & also get the correct format and handle if the
timestamps are nulls to ""?

thanks