Avoid infinite loop in lexer on unclosed code block

lib/lexer_mdx.mll

@@ -56,7 +56,8 @@ rule text section = parse
         `Text str :: text section lexbuf }
 
 and block = parse
-  | eof | ws* as end_pad "```" ws* eol
+  | eof { [""] }
+  | ws* as end_pad "```" ws* eol
     { newline lexbuf;
       [end_pad] }
   | ([^'\n']* as str) eol

test/bin/mdx-test/expect/dune.inc

@@ -575,6 +575,18 @@
  (alias runtest)
  (action (diff trailing-whitespaces/test-case.md.expected trailing-whitespaces.actual)))
 
+(rule
+ (target unclosed-block.actual)
+ (deps (package mdx) (source_tree unclosed-block))
+ (action
+  (with-stdout-to %{target}
+   (chdir unclosed-block
+    (run ocaml-mdx test --output - test-case.md)))))
+
+(rule
+ (alias runtest)
+ (action (diff unclosed-block/test-case.md.expected unclosed-block.actual)))
+
 (rule
  (target warnings.actual)
  (deps (package mdx) (source_tree warnings))

test/bin/mdx-test/expect/unclosed-block/test-case.md

@@ -0,0 +1,7 @@
+Here is a good block:
+
+```ocaml
+```
+
+And here is one that is not closed, this should not cause an infinite loop in the lexer:
+```

test/bin/mdx-test/expect/unclosed-block/test-case.md.expected

@@ -0,0 +1,8 @@
+Here is a good block:
+
+```ocaml
+```
+
+And here is one that is not closed, this should not cause an infinite loop in the lexer:
+```
+```