[RavSinghChandan]

With the footnotes extra, a footnote that is defined but never referenced still makes markdown2 append the footnotes block, with an empty list:

>>> markdown2.markdown("Text.\n\n[^1]: A note.\n", extras=["footnotes"])
'<p>Text.</p>\n\n<div class="footnotes">\n<hr />\n<ol>\n</ol>\n</div>\n'

_add_footnotes checks self.footnotes (the definitions) but only lists self.footnote_ids (the referenced ones), so the block is empty when nothing is referenced. It now checks the referenced ids, so the output is just <p>Text.</p>. Documents with referenced footnotes are unchanged, and unreferenced definitions were already left out of the list.

Added the footnotes_unreferenced test case