🩹 Guard exception definition when disabled

When the preprocessor token `NOT_NULL_EXCEPTIONS_DISABLED` is defined,
all exception-related code -- including the exception itself -- should
no longer be emitted. However, a bug in the implementation is
resulting in the _definition_ of the constructor to be present, while
the class declaration is not present -- resulting in compile-errors.

This guards the constructor body definition as well so that this will
not occur.

include/not_null.hpp

@@ -641,6 +641,8 @@ inline namespace bitwizeshift {
 } // inline namespace bitwizeshift
 } // namespace cpp
 
+#if !defined(NOT_NULL_DISABLE_EXCEPTIONS)
+
 inline
 NOT_NULL_NS_IMPL::not_null_contract_violation::not_null_contract_violation()
   : logic_error{
@@ -651,6 +653,8 @@ NOT_NULL_NS_IMPL::not_null_contract_violation::not_null_contract_violation()
 
 }
 
+#endif // !defined(NOT_NULL_DISABLE_EXCEPTIONS)
+
 //=============================================================================
 // detail utilities : not_null
 //=============================================================================