Avoid trying to find max between None and non None value

[pvonglehn]

Description of change

fixes #19

Set the value of max_bookmark to the first value of the bookmark column directly without comparing the first value of the bookmark column to None. This avoids TypeError from max(None, 1)

Manual QA steps

Set up a new tap project with non None bookmark column and run the tap

Risks

Rollback steps

• revert this branch

[pvonglehn]

@nick-mccoy @dmosorast @aaronsteers could you kindly review or let me know who can, thanks!