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/Storage/StorageManagement.cpp

@@ -90,16 +90,16 @@ OwnPtr<PartitionTable> StorageManagement::try_to_initialize_partition_table(cons
     if (mbr_table_or_result.error() == PartitionTable::Error::MBRProtective) {
         auto gpt_table_or_result = GUIDPartitionTable::try_to_initialize(device);
         if (gpt_table_or_result.is_error())
-            return nullptr;
+            return {};
         return move(gpt_table_or_result.value());
     }
     if (mbr_table_or_result.error() == PartitionTable::Error::ConatinsEBR) {
         auto ebr_table_or_result = EBRPartitionTable::try_to_initialize(device);
         if (ebr_table_or_result.is_error())
-            return nullptr;
+            return {};
         return move(ebr_table_or_result.value());
     }
-    return nullptr;
+    return {};
 }
 
 NonnullRefPtrVector<DiskPartition> StorageManagement::enumerate_disk_partitions() const