Refactor: Store null instead of empty string for optional employee fields in the DB

The parseEmployeeFormData function is written in such a way that we do not store null for empty/untouched employee fields in the database.

However, in the handleEditOffice, we do
```
const notes = formData.get("notes") as string || null;
```
So, we do store null here.

Hence, I need to make this consistent across. Make sure I store null for optional fields in employee as well in the DB