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/Partition/MBRPartitionTable.cpp

@@ -54,7 +54,7 @@ OwnPtr<MBRPartitionTable> MBRPartitionTable::try_to_initialize(const StorageDevi
 {
     auto table = make<MBRPartitionTable>(device, start_lba);
     if (!table->is_valid())
-        return nullptr;
+        return {};
     return table;
 }