Parsing error with mapping processors that have a block of Bloblang in them which starts with an empty line

Trying to run the following config:

```yaml
input:
  generate:
    count: 1
    mapping: root.foo = "bar"
  processors:
    - mapping: |

        root.bar = "baz"
```

triggers this error:
> failed to generate sanitised config: yaml: line 24: did not find expected key

If we remove the empty newline from the `mapping` block, it works as expected.