PSM I - Few clarifications required

Hi,

Can you please help me finding answers for the below questions?

1) What is a Trendline on a Release Burndown chart indicate? How does it help the Scrum team?

2) Can the Development team decide or recommend the Scrum master not to have the Sprint Retrospective meeting to be held, when they feel there is nothing that can be discussed?

I understand that the other Scrum events like Sprint Planning, Daily Scrum and Sprint Review meetings can't be skipped as their purpose seems critical in a Sprint.

3) What are the responsibilities of a tester in a Development team?

4) Should a properly functioning scrum team have at least one Release in a Sprint?

5) Is the Product owner responsible for maintaining the Product burndown chart? I understand the Development team owns the Sprint backlog and the Sprint burndown chart as well. Kindly explain.

Thanks!