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/CoreDump.cpp

@@ -45,12 +45,12 @@ OwnPtr<CoreDump> CoreDump::create(NonnullRefPtr<Process> process, const String&
 {
     if (!process->is_dumpable()) {
         dbgln("Refusing to generate CoreDump for non-dumpable process {}", process->pid().value());
-        return nullptr;
+        return {};
     }
 
     auto fd = create_target_file(process, output_path);
     if (!fd)
-        return nullptr;
+        return {};
     return adopt_own(*new CoreDump(move(process), fd.release_nonnull()));
 }