span: Add Span::empty() and use it in script/descriptor

src/span.h

@@ -151,6 +151,7 @@ public:
         return m_data[m_size - 1];
     }
     constexpr std::size_t size() const noexcept { return m_size; }
+    constexpr bool empty() const noexcept { return size() == 0; }
     CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
     {
         ASSERT_IF_DEBUG(size() > pos);