LibWeb: Return Optional from StyleProperties::box_sizing()

This function was written as if it returned `Optional<CSS::BoxSizing>`
but actually returned a plain `CSS::BoxSizing`, meaning if the property
was not set or was invalid, it would return whichever enum value was
first. This wasn't visible because we don't yet pay any attention to
the `box-sizing` property.

Userland/Libraries/LibWeb/CSS/StyleProperties.cpp

@@ -961,7 +961,7 @@ Vector<ShadowData> StyleProperties::text_shadow() const
     return shadow(PropertyID::TextShadow);
 }
 
-CSS::BoxSizing StyleProperties::box_sizing() const
+Optional<CSS::BoxSizing> StyleProperties::box_sizing() const
 {
     auto value = property(CSS::PropertyID::BoxSizing);
     if (!value.has_value())