chore: harden sanitizer null-safety for codeblock content

Agent-Logs-Url: https://github.com/statch/gitbot/sessions/71de78f4-95f9-4b00-bbde-4a621af540f1

Co-authored-by: seven7ty <63970738+seven7ty@users.noreply.github.com>

Author: Copilot

lib/manager.py

@@ -232,15 +232,17 @@ def truncate(str_: str, length: int, ending: str = '...', full_word: bool = Fals
         return str_
 
     @staticmethod
-    def sanitize_codeblock_content(content: str, neutralize_mentions: bool = True) -> str:
+    def sanitize_codeblock_content(content: str | None, neutralize_mentions: bool = True) -> str:
         """
         Harden untrusted text for Discord fenced code blocks.
 
         :param content: The text to sanitize
         :param neutralize_mentions: Whether to prevent mention abuse
         :return: The sanitized text
         """
-        content: str = content.replace('```', '`\u200b``')
+        if not content:
+            return ''
+        content = content.replace('```', '`\u200b``')
         if neutralize_mentions:
             content = content.replace('@', '@\u200b')
         return content