Markdown reader: allow empty code spans.

E.g. `` ` ` ``.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1591,7 +1591,7 @@ code = try $ do
   starts <- many1 (char '`')
   skipSpaces
   result <- (trim . concat) <$>
-            many1Till (many1 (noneOf "`\n") <|> many1 (char '`') <|>
+            manyTill (many1 (noneOf "`\n") <|> many1 (char '`') <|>
                        (char '\n' >> notFollowedBy' blankline >> return " "))
                       (try (skipSpaces >> count (length starts) (char '`') >>
                       notFollowedBy (char '`')))

test/command/empty-inline-code.txt

@@ -0,0 +1,6 @@
+```
+% pandoc -t native
+` `
+^D
+[Code ("",[],[]) ""]
+```