fix the bugs when some parameters are empty

[zjasper666]

trafficstars

Why are these changes needed?

Fix the bugs when sampling_weights is empty; Fix the bugs when random_questions is empty

Related issue number (if applicable)

Checks

• [x] I've run format.sh to lint the changes in this PR.
• [x] I've included any doc changes needed.
• [x] I've made sure the relevant tests are passing (if applicable).