Fix error when passed "00:00:00"

[ooyendyk]

When passing the argument "00:00:00", visual_timer passes the following error:

Traceback (most recent call last): File "visual_timer.py", line 45, in <module> y_sec_unit = timer_functions.divide_y(timer_set, dimensions[0]) File "/home/user/Documents/Code/visual_timer/visual_timer/timer_functions.py", line 46, in divide_y y_sec_unit = y_dim/timer_set ZeroDivisionError: division by zero