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/LibCore/Event.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/WeakPtr.h>
 #include <LibCore/Event.h>
 #include <LibCore/Object.h>
 
@@ -32,7 +33,7 @@ namespace Core {
 ChildEvent::ChildEvent(Type type, Object& child, Object* insertion_before_child)
     : Core::Event(type)
     , m_child(child.make_weak_ptr())
-    , m_insertion_before_child(insertion_before_child ? insertion_before_child->make_weak_ptr() : nullptr)
+    , m_insertion_before_child(AK::try_make_weak_ptr(insertion_before_child))
 {
 }