Correct mailmapping of empty email addresses

`not old_email` doesn't distinguish between `None` and an empty string,
causing old emails specified as `<>` to apply to every single commit.

Signed-off-by: Riley Iverson <blepabyte@proton.me>

git-filter-repo

@@ -320,8 +320,8 @@ class MailmapInfo(object):
     for old, new in self.changes.items():
       old_name, old_email = old
       new_name, new_email = new
-      if (not old_email or email.lower() == old_email.lower()) and (
-          name  == old_name or not old_name):
+      if (old_email is None or email.lower() == old_email.lower()) and (
+          name == old_name or not old_name):
         return (new_name or name, new_email or email)
     return (name, email)