AK: Simplify constructors and conversions from nullptr_t

Problem:
- Many constructors are defined as `{}` rather than using the ` =
  default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
  instead of requiring the caller to default construct. This violates
  the C++ Core Guidelines suggestion to declare single-argument
  constructors explicit
  (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).

Solution:
- Change default constructors to use the compiler-provided default
  constructor.
- Remove implicit conversion operators from `nullptr_t` and change
  usage to enforce type consistency without conversion.

Libraries/LibWeb/HTML/CanvasRenderingContext2D.cpp

@@ -142,11 +142,11 @@ void CanvasRenderingContext2D::did_draw(const Gfx::FloatRect&)
 OwnPtr<Gfx::Painter> CanvasRenderingContext2D::painter()
 {
     if (!m_element)
-        return nullptr;
+        return {};
 
     if (!m_element->bitmap()) {
         if (!m_element->create_bitmap())
-            return nullptr;
+            return {};
     }
 
     return make<Gfx::Painter>(*m_element->bitmap());
@@ -208,7 +208,7 @@ RefPtr<ImageData> CanvasRenderingContext2D::create_image_data(int width, int hei
 {
     if (!wrapper()) {
         dbgln("Hmm! Attempted to create ImageData for wrapper-less CRC2D.");
-        return nullptr;
+        return {};
     }
     return ImageData::create_with_size(wrapper()->global_object(), width, height);
 }