FindClosestBlock incorrectly returns LinkReferenceDefinitionGroup

FindClosestBlock incorrectly returns LinkReferenceDefinitionGroup when:

• Markdown document ends with a list,
• AutoIdentifierExtension is active and provides the LinkReferenceDefinitionGroup
• I call MarkdownDocument.FindClosestBlock() passing in line number of "Second list item" - hoping to get the second ListItemBlock

Example document, parsed with Markdig 0.30.4

Paragraph
 - First list item with a [link](target.txt)
 - Second list item with a [link](target.txt) <eod>

Associated MarkdownDocument has the following items at the end of the blocks collection:

ParapgraphBlock
ListBlock
LinkReferenceDefinitionGroup

MarkdownDocument.FindClosestBlock() incorrectly determines that the closest block is LinkReferenceDefinitionGroup, because it happens to be the last item in the blocks.
LinkReferenceDefinitionGroup should not be returned as it does not even exist in the actual document: LinkReferenceDefinitionGroup.Span is {0 - -1}, it IsEmpty and it's Line is 0.

This issue doesn't happen if I call MarkdownDocument.FindClosestBlock() passing in line number of "First list item" which directly matches the ListBlock, or if I type underneath the list, which places ParagraphBlock underneath ListBlock, and prevents the binary search from stumbling upon LinkReferenceDefinitionGroup.

Start at MarkdownDocument.FindClosestBlock and reproduce the example with AutoIdentifierExtension enabled, ending the document with the list. Verify how the empty LinkReferenceDefinitionGroup affects the block search; done means the second list item's line resolves to the ListBlock or ListItemBlock rather than the empty group, while the other described cases still work.