Fix edge-case crash in codehilite

If there is an empty `<pre><code></code></pre>` inserted by another extension, there can be an exception. The added test case was crashing before this change.

docs/changelog.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Remove legacy import needed only in Python 2 (#1403)
 * Fix typo that left the attribute `AdmonitionProcessor.content_indent` unset
   (#1404)
+* Fix edge-case crash in `codehilite` with an empty `code` tag (#1405).
 * Improve and expand type annotations in the code base (#1401).
 
 ## [3.5.1] -- 2023-10-31

markdown/extensions/codehilite.py

@@ -270,8 +270,11 @@ def run(self, root: etree.Element) -> None:
         for block in blocks:
             if len(block) == 1 and block[0].tag == 'code':
                 local_config = self.config.copy()
+                text = block[0].text
+                if text is None:
+                    continue
                 code = CodeHilite(
-                    self.code_unescape(block[0].text),
+                    self.code_unescape(text),
                     tab_length=self.md.tab_length,
                     style=local_config.pop('pygments_style', 'default'),
                     **local_config

tests/test_syntax/extensions/test_code_hilite.py

@@ -21,7 +21,9 @@
 
 from markdown.test_tools import TestCase
 from markdown.extensions.codehilite import CodeHiliteExtension, CodeHilite
+from markdown import extensions, treeprocessors
 import os
+import xml.etree.ElementTree as etree
 
 try:
     import pygments  # noqa
@@ -762,3 +764,22 @@ def testFormatterLangStrEmptyLang(self):
                 )
             ]
         )
+
+    def testDoesntCrashWithEmptyCodeTag(self):
+        expected = '<h1>Hello</h1>\n<pre><code></code></pre>'
+        self.assertMarkdownRenders(
+            '# Hello',
+            expected,
+            extensions=[CodeHiliteExtension(), _ExtensionThatAddsAnEmptyCodeTag()]
+        )
+
+
+class _ExtensionThatAddsAnEmptyCodeTag(extensions.Extension):
+    def extendMarkdown(self, md):
+        md.treeprocessors.register(_AddCodeTagTreeprocessor(), 'add-code-tag', 40)
+
+
+class _AddCodeTagTreeprocessor(treeprocessors.Treeprocessor):
+    def run(self, root: etree.Element):
+        pre = etree.SubElement(root, 'pre')
+        etree.SubElement(pre, 'code')