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.

Userland/test.cpp

@@ -345,7 +345,7 @@ static OwnPtr<Condition> parse_simple_expression(char* argv[])
 {
     StringView arg = argv[optind];
     if (arg.is_null()) {
-        return nullptr;
+        return {};
     }
 
     if (arg == "(") {
@@ -403,7 +403,7 @@ static OwnPtr<Condition> parse_simple_expression(char* argv[])
             // '-a' and '-o' are boolean ops, which are part of a complex expression
             // so we have nothing to parse, simply return to caller.
             --optind;
-            return nullptr;
+            return {};
         case 'n':
             return make<StringCompare>("", value, StringCompare::NotEqual);
         case 'z':