[YAML] fix handling of empty sequence items

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11798
| License       | MIT
| Doc PR        |

When a line contains only a dash it cannot safely be assumed that it contains a nested list or an embedded mapping. If the next line starts with a dash at the same indentation, the current line's item is to be treated as `null`.