linux command to empty all files of a directory

This will do what you want:

for f in *; do >$f; done

You can't use redirection (>) within find -exec directly because it happens before the command runs and creates a file called {}. To get around this you need to do it in a new shell by using sh -c.

Also, note that you don't need to cat /dev/null > file in order to clobber a file. You can simply use > file.

Try this:

find . -type f -exec sh -c '>"{}"' \;