What if I've got nothing to say?
Then write that, and describe the last hour in plain detail. 'Nothing to say' entries are often the most revealing ones in the reread.
Should I write for a future reader?
No. The moment you imagine an audience, you start performing and the honesty drains out. Write like the notebook will be burned.
Do I have to reread it?
Yes, if you want it to be useful rather than just soothing. The weekly review is where journaling turns into feedback.
Is it a problem if I only ever vent?
Venting nightly about the same thing without moving can deepen the groove. Add the 'and what would I do about it?' line, and if it's the same grievance for months, that may be worth taking to a person rather than a page.