lastRun should return undefined on first run

[fabpopa]

Hi, I think I'm seeing an issue with Gulp 4.0 after the dependency for undertaker was bumped from ^0.7.0 to ^0.11.0 a week ago. My tasks using lastRun aren't building, since

gulp.task('build:scripts', function() {
  console.log(gulp.lastRun('build:scripts'));    // this outputs the timestamp on first run, not undefined
  return gulp.src('app/**/*.js', { since: gulp.lastRun('build:scripts') })
    .pipe(gulp.dest('dist/app'));
});

outputs the timestamp even on first run, so no change is registered in the files.
Is this just me? Thank you.

[phated]

Can you write a failing test for this and submit as a PR? The current tests are all passing.

[phated]

Fixed by 96540e3