[misc] Fix previous change to <pre><code> (#18312)

Turns out that `<pre>...</pre>` blocks ignore the first empty line, but
`<pre><code>...</code></pre>` blocks don't. So if we put the first real
line of code on the html line after the tags, it will render as an empty
line. (a problem that didn't exist for just pre tags) Let's remove those
extra newlines after code tags.

(I still think it's nice to have code tags for future syntax
highlighting on the blog posts)

Author: svalentin

misc/gen_blog_post_html.py

@@ -62,7 +62,9 @@ def format_code(h: str) -> str:
         else:
             r.append(a[i])
             i += 1
-    return "\n".join(r)
+    formatted = "\n".join(r)
+    # remove empty first line for code blocks
+    return re.sub(r"<code>\n", r"<code>", formatted)
 
 
 def convert(src: str) -> str: