fix: avoid lower casing none

Author: mvanholsteijn

src/wordpress_markdown_blog_loader/api.py

@@ -338,7 +338,7 @@ def get_unique_user_by_name(self, name: str, email: Optional[str]) -> "User":
         if len(users) == 0:
             raise ValueError(f"author '{name}' not found on {self.endpoint.host}")
         elif len(users) > 1:
-            user = next(filter(lambda u: email and u.email.lower() == email.lower(), users), None)
+            user = next(filter(lambda u: email and u.email and u.email.lower() == email.lower(), users), None)
             if not user:
                 raise ValueError(
                     f"Multiple authors named '{name}' found, none with email {email} (possible: { {u.email for u in users} })."