[blog] tag_text with HTML inside adds empty <p>

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If I use a tag_text with, for example, <h1>Tag Name</h1>, it works as I expect, but there's a spurious empty <p> tag, causing strange spacing issues.

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#258 might fix this.

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Yes, this is absolutely because the template puts it in a <p> unnecessarily. We fixed #258, so now we need to add this, but also test to ensure that it works correctly (using t/theme/check.t).