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/CSS/Length.h

@@ -35,7 +35,7 @@ public:
         Vmin,
     };
 
-    Length() { }
+    Length() = default;
     Length(int value, Type type)
         : m_type(type)
         , m_value(value)