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/EventLoop.cpp

@@ -503,7 +503,7 @@ bool SignalHandlers::remove(int handler_id)
         auto it = m_handlers.find(handler_id);
         if (it != m_handlers.end()) {
             // Mark pending remove
-            m_handlers_pending.set(handler_id, nullptr);
+            m_handlers_pending.set(handler_id, {});
             return true;
         }
         it = m_handlers_pending.find(handler_id);