Fix division by 0 issue when all scores are zero

[AlanKerstjens]

In graph_ga/goal_directed_generation.py the mating pool is selected through weighted random sampling, with the probability of a molecule being selected given by its score divided by the sum of all scores (sum_scores).

https://github.com/BenevolentAI/guacamol_baselines/blob/44d24c53f3acf9266eb2fb06dbff909836549291/graph_ga/goal_directed_generation.py#L25-L42

If sum_scores == 0.0 we get a division by 0. This actually occurs during the Valsartan SMARTS benchmark when using random starting molecules due to its binary nature.