_repr_helper of MongoClient mishandles socketTimeoutMS=None

Hello,

According to the docs, one can set/leave socketTimeoutMS as None in order to not have a timeout. However, when you try to log/print any database/collection/client having this, you'll get a TypeError.

The code here https://github.com/mongodb/mongo-python-driver/blob/master/pymongo/mongo_client.py#L842-L843 tries to format the socketTimeoutMS value, which is None, and can't be multiplied by 1000.

Best,
Mihnea