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/WeakPtr.h

@@ -37,8 +37,7 @@ class WeakPtr {
     friend class Weakable;
 
 public:
-    WeakPtr() { }
-    WeakPtr(std::nullptr_t) { }
+    WeakPtr() = default;
 
     template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
     WeakPtr(const WeakPtr<U>& other)
@@ -261,6 +260,15 @@ struct Formatter<WeakPtr<T>> : Formatter<const T*> {
     }
 };
 
+template<typename T>
+WeakPtr<T> try_make_weak_ptr(const T* ptr)
+{
+    if (ptr) {
+        return ptr->template make_weak_ptr<T>();
+    }
+    return {};
+}
+
 }
 
 using AK::WeakPtr;