Edit sample_mlacomp input checks regarding outfile

If outfile was specified as NULL ss3sim would still run but an error
message would appear, Error in if (substr_r(outfile, 4) != ".dat" &
write_file) stop(paste0("outfile ",  : argument is of length zero,
the fix makes sure that outfile is not null prior to checking to see
if the argument ends in .dat.