Empty coordinates in commands are allowed, and will be interpreted as 0 0 0

Empty coordinates in commands are allowed, and will be interpreted as 0 0 0. Do note that this does not work if the commands are executed via chat, because it also trims spaces in between values. No error is thrown if you just paste the command, though.

particle barrier       1 1 force
teleport @a    facing ~ ~ ~
clone          replace force

particle barrier       1 1 force
teleport @a    facing ~ ~ ~
clone          replace force

This does not seem to happen if you do not include any additional values. The following commands are marked as incorrect: (This will not work in functions, because spaces get trimmed at the end of commands. See MC-198110)

particle barrier       
teleport @a    
clone

particle barrier       
teleport @a    
clone