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/KBuffer.h

@@ -128,7 +128,7 @@ public:
     {
         auto impl = KBufferImpl::try_create_with_size(size, access, name, strategy);
         if (!impl)
-            return nullptr;
+            return {};
         return adopt_own(*new KBuffer(impl.release_nonnull()));
     }
 
@@ -136,7 +136,7 @@ public:
     {
         auto impl = KBufferImpl::try_create_with_bytes(bytes, access, name, strategy);
         if (!impl)
-            return nullptr;
+            return {};
         return adopt_own(*new KBuffer(impl.release_nonnull()));
     }