LibWeb: Add null check for get_client_rects() on non paintable elements

Author: https://github.com/Cnidarias
Commit: 1a1e5aaf54
Pull-request: https://github.com/SerenityOS/serenity/pull/23990
Issue: https://github.com/SerenityOS/serenity/issues/23829

Tests/LibWeb/Text/expected/get-bounding-client-rect-display-none.txt

@@ -1 +1,3 @@
+ 
+   {"x":0,"y":0,"width":0,"height":0,"top":0,"right":0,"bottom":0,"left":0}
 {"x":0,"y":0,"width":0,"height":0,"top":0,"right":0,"bottom":0,"left":0}

Tests/LibWeb/Text/input/get-bounding-client-rect-display-none.html

@@ -3,10 +3,14 @@
     #box { display: none; }
 </style>
 <div id="box"></div>
+<br id="br"/>
 <script src="include.js"></script>
 <script>
     test(() => {
         const rect = document.getElementById("box").getBoundingClientRect();
         println(JSON.stringify(rect));
+
+        const br_rect = document.getElementById("br").getBoundingClientRect();
+        println(JSON.stringify(br_rect));
     });
 </script>

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -884,16 +884,16 @@ JS::NonnullGCPtr<Geometry::DOMRectList> Element::get_client_rects() const
     const_cast<Document&>(document()).update_paint_and_hit_testing_properties_if_needed();
 
     Gfx::AffineTransform transform;
-    if (auto const* paintable_box = this->paintable_box())
-        transform = Gfx::extract_2d_affine_transform(paintable_box->transform());
     CSSPixelPoint scroll_offset;
-    for (auto const* containing_block = paintable()->containing_block(); containing_block; containing_block = containing_block->containing_block()) {
+    auto const* paintable = this->paintable();
+
+    if (auto const* paintable_box = this->paintable_box()) {
+        transform = Gfx::extract_2d_affine_transform(paintable_box->transform());
+        for (auto const* containing_block = paintable->containing_block(); containing_block; containing_block = containing_block->containing_block()) {
             transform = Gfx::extract_2d_affine_transform(containing_block->transform()).multiply(transform);
             scroll_offset.translate_by(containing_block->scroll_offset());
         }
 
-    auto const* paintable = this->paintable();
-    if (auto const* paintable_box = this->paintable_box()) {
         auto absolute_rect = paintable_box->absolute_border_box_rect();
         auto transformed_rect = transform.map(absolute_rect.translated(-paintable_box->transform_origin()).to_type<float>())
                                     .to_type<CSSPixels>()