This fixes TNL-7973 [1].
Background
----------
> An alert was sent out because a malformed capa problem caused block
> transformers to fail to run.
> This specific error/alert was triggered when an optionresponse problem
> lacked text. The authoring-based fix is to edit the offending
> option-response problem to remove the empty option.
> Expected behavior: We should be able to keep the error local to the
> ProblemBlock and not blow up the whole course publish block
> transformer collection process.
> This is a really easy error for authors to make, and the consequences
> to them (one problem doesn’t work) are disproportionate to the
> consequences [...] (alerting triggered).
- [1] https://openedx.atlassian.net/browse/TNL-7973
21dce731f9