If an assessment has 100 questions and the respondent logs in and gets to the first page, are records created for all 100 questions of the assessment right away? So could someone who started an assessment, but then closes it, have a 100 question records with empty answers?

Yes you are correct, assuming it is a non-Branching assessment.  Regular assessments will generate all “Result” table records upon login.  Branching assessments, on the other hand, will only generate the Result records up until it encounters a Branching style question, and then it will stop.  Once the respondent answers the branching question, the system will generate the rest of the assessment (or until it encounters another branching style question).