Stop Validators from Sending Empty Challenges

[bkb2135]

For wiki-based tasks, validators send empty challenges ~1.2% of the time.

[bkb2135]

If the challenge is empty, we should just raise an exception and try again/skip the task. We should check this after we generate the challenge and clean it.

[frozenmafia]

Hi @bkb2135 I have raised PR for this issue .