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.

AK/SinglyLinkedList.h

@@ -37,7 +37,7 @@ namespace AK {
 template<typename ListType, typename ElementType>
 class SinglyLinkedListIterator {
 public:
-    SinglyLinkedListIterator() { }
+    SinglyLinkedListIterator() = default;
     bool operator!=(const SinglyLinkedListIterator& other) const { return m_node != other.m_node; }
     SinglyLinkedListIterator& operator++()
     {
@@ -78,7 +78,7 @@ private:
     };
 
 public:
-    SinglyLinkedList() { }
+    SinglyLinkedList() = default;
     ~SinglyLinkedList() { clear(); }
 
     bool is_empty() const { return !head(); }