Fix issue raised by the mypy hook

The `mypy` hook flagged trying to access an element of an Optional
value so we must perform a None check first.

Author: mcdonnnj

src/hash_http_content/hasher.py

@@ -140,7 +140,7 @@ def _is_visible_element(self, element: PageElement) -> bool:
         if isinstance(element, Comment):
             logging.debug("Skipping Comment tag")
             return False
-        if element.parent.name in discard_tags:
+        if element.parent is not None and element.parent.name in discard_tags:
             logging.debug("Skipping element in parent tag '%s'", element.parent.name)
             return False
         return True