Fix example extraction

Assumtion was that the first and last lines of examples are always
blank. In 2021/06 the example is only one line. It has no blank line at
the end so the example was deleted by the script.

Changed:
The script will now check if the line is actually blank before deleting
it. Requires that the indentation is removed beforehand.

lib/aoc.sh

@@ -26,12 +26,12 @@ echo "Extracting example code blocks..."
 # extract example code blocks
 cat README.md |
   sed -n '/\[code\]/,/\[\/code\]/p' | # extract code blocks
-  sed '/\[code\]/{n;d}' | # remove (blank) lines after opening bracket
+  sed 's/^    //' | # remove indentation
+  sed '/\[code\]/{n;/^$/d}' | # remove blank lines after opening bracket
   tac | # reverse line order
-  sed '/\[\/code\]/{n;d}' | # remove (blank) lines after closing brackets (preceding blank lines in normal order)
+  sed '/\[\/code\]/{n;/^$/d}' | # remove blank lines after closing brackets (preceding blank lines in normal order)
   tac | # reverse line order again
   sed '/^\[\/code\]/d' | # remove closing brackets
-  sed 's/^    //' | # remove indentation
   csplit - --suppress-matched --elide-empty-files --prefix='input/example' --suffix='%d' '/\[code\]/' '{*}'
 echo "done."