time: Fix strftime(3) API regarding nullability

strftime(3) doesn't accept null pointers in any of the parameters.

Cc: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>