fix(markdown): Ensure content is followed by '#'

Before this commit, empty headings are taken into account:

   # a normal heading

   below is not a good heading:

   #

   below is also not a good heading:

   ##

   These should not be listed in the outline.

README.md

@@ -47,6 +47,8 @@ I do not merge PRs from the original repo that I don't personally need.
 - Open handler triggering multiple times ends up in messy state with errors
   simrat39/symbols-outline.nvim#235
 - Fixed `_highlight_current_item` function checking provider on outline window
+- Fixed behaviour of empty markdown headings `^(#+)(%s+)$` being listed in the
+outline.
 
 ## 🛑 Breaking changes