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/LibPCIDB/Database.cpp

@@ -138,10 +138,10 @@ int Database::init()
 
     ParseMode mode = ParseMode::UnknownMode;
 
-    OwnPtr<Vendor> current_vendor = nullptr;
-    OwnPtr<Device> current_device = nullptr;
-    OwnPtr<Class> current_class = nullptr;
-    OwnPtr<Subclass> current_subclass = nullptr;
+    OwnPtr<Vendor> current_vendor {};
+    OwnPtr<Device> current_device {};
+    OwnPtr<Class> current_class {};
+    OwnPtr<Subclass> current_subclass {};
 
     auto commit_device = [&]() {
         if (current_device && current_vendor) {