Issue rendering code blocks with triple ticks.
As seen in this message: https://forum.nim-lang.org/t/7926#51699

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Couldn't render historic post in #51699.
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Editing preview: `Message needs to be valid RST! Error: input(46, 1) Error: new section expected (underline '```' is too short)` The edited post is not saveable, unfortunately (#315).

The lack of an empty line before the opening triple ticks seems to be the problem. 

**Desired behaviour:** not needing an empty line before opening a new code block.