Fix Timer.fmt for short result strings

[nliviu]

When doing eg. Timer.fmt('%H', Timer.now()) the resulting string is .
Test code:

    let now = Timer.now();
    let hour = Timer.fmt("%H", now);
    print('++++', JSON.stringify({hour: hour}));

Original Timer.fmt output:

{"hour":"  "}

Modified:

{"hour":"20"}