support pandoc3 - return empty list instead of Null

[dmitryperets]

On pandoc 3.1, the filter fails with the following error:

Error running filter /common/filters/pandoc-comments.py:
Error in $.blocks[254]: When parsing Text.Pandoc.Definition.Block expected an Object with a tag field where the value is one of [Plain, Para, LineBlock, CodeBlock, RawBlock, BlockQuote, OrderedList, BulletList, DefinitionList, Header, HorizontalRule, Table, Figure, Div], but got Null.

To fix it, I return [] instead of Null(), which is also according to the examples from pandocfilters repo.

[TimMoser92]

Works great. Thank you!