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.

Kernel/Thread.h

@@ -207,10 +207,6 @@ public:
             : m_infinite(true)
         {
         }
-        BlockTimeout(std::nullptr_t)
-            : m_infinite(true)
-        {
-        }
         explicit BlockTimeout(bool is_absolute, const timeval* time, const timespec* start_time = nullptr, clockid_t clock_id = CLOCK_MONOTONIC_COARSE)
             : m_clock_id(clock_id)
             , m_infinite(!time)