Timesheet API: Fix up updating database

[izzyconner]

We essentially need to make sure when we are updating the database for timesheet updates that we are only changing fields/attributes that actually have changes made, rather than reupdating the entire row (which is what we are doing rn).

See the slack thread for context: https://c4cneuteams.slack.com/archives/C043TJM55BN/p1682523526570869