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/expr.cpp

@@ -370,7 +370,7 @@ public:
         Match,
     };
 
-    StringExpression(StringOperation op, NonnullOwnPtr<Expression> string, OwnPtr<Expression> pos_or_chars = nullptr, OwnPtr<Expression> length = nullptr)
+    StringExpression(StringOperation op, NonnullOwnPtr<Expression> string, OwnPtr<Expression> pos_or_chars = {}, OwnPtr<Expression> length = {})
         : m_op(op)
         , m_str(move(string))
         , m_pos_or_chars(move(pos_or_chars))