fix: avoid crashing if an assay is missing comment

[thiswillbeyourgithub]

Signed-off-by: thiswillbeyourgithub 26625900+thiswillbeyourgithub@users.noreply.github.com

Here is the code that was crashing before:

import json
import pubchempy as pcp

assays = pcp.get_assays([2062])

for a in assays:
    print(a.to_dict())