LibWeb: Don't try to invalidate style for character data nodes

Character data nodes like text and HTML comments don't have style,
so let's just exit invalidation immediately for those.

(cherry picked from commit f351f75a34619ad969f660312a2e5486f742f685)

Userland/Libraries/LibWeb/DOM/Node.cpp

@@ -389,6 +389,9 @@ JS::GCPtr<HTML::Navigable> Node::navigable() const
 
 void Node::invalidate_style(StyleInvalidationReason reason)
 {
+    if (is_character_data())
+        return;
+
     if (!needs_style_update() && !document().needs_full_style_update()) {
         dbgln_if(STYLE_INVALIDATION_DEBUG, "Invalidate style ({}): {}", to_string(reason), debug_description());
     }