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/LibJS/Runtime/Uint8ClampedArray.cpp

@@ -42,7 +42,7 @@ Uint8ClampedArray::Uint8ClampedArray(u32 length, Object& prototype)
     , m_length(length)
 {
     auto& vm = this->vm();
-    define_native_property(vm.names.length, length_getter, nullptr);
+    define_native_property(vm.names.length, length_getter, {});
     m_data = (u8*)calloc(m_length, 1);
 }