Why would you want to multiply by 1 or sum zero? You probably wouldn't need to do it, but computers and scripts aren't so smart as people and always follow code literally. For them is important a language having less irregularities and exceptional rules. If "" is a valid command and you have the syntax "time [COMMAND];", why not allow "time;"?

It might be to avoid errors for scripts like:

  time ${cmd}; foo

where cmd happens to be unset. Conceptually, that's a valid script, because `time` standalone is a valid script. No reason it should generate an error.

Not always the case, I just tried and:

$ time

real 0m0.002s user 0m0.000s sys 0m0.000s