LibWeb: Use default instead of an empty constructor/destructor

Default implementations allow for more optimizations.
See: https://pvs-studio.com/en/docs/warnings/v832/

Userland/Libraries/LibWeb/DOM/NonDocumentTypeChildNode.h

@@ -47,7 +47,7 @@ public:
     const Element* next_element_in_pre_order() const { return const_cast<NonDocumentTypeChildNode*>(this)->next_element_in_pre_order(); }
 
 protected:
-    NonDocumentTypeChildNode() { }
+    NonDocumentTypeChildNode() = default;
 };
 
 }

Userland/Libraries/LibWeb/DOM/NonElementParentNode.h

@@ -34,7 +34,7 @@ public:
     }
 
 protected:
-    NonElementParentNode() { }
+    NonElementParentNode() = default;
 };
 
 }