500 error when using a markdown task list item across lines

As reported in #421985 (comment 1632753686), a certain task list item can cause a problem:

- one
  - foo
    [ ] bar

This causes the error

NoMethodError - undefined method `captures' for nil:NilClass:
  lib/banzai/filter/task_list_filter.rb:90:in `call'
  lib/banzai/pipeline/base_pipeline.rb:23:in `block (2 levels) in singleton class'