Add missing is_set() implementation.

src/env_real.cpp

@@ -263,6 +263,17 @@ void EnvReal::unset (std::string_view name) noexcept {
 }
 
 bool EnvReal::is_set (std::string_view name) const noexcept {
+	auto& pm = pointer_map();
+	std::lock_guard<std::mutex> lock(pm.env_mutex());
+
+	return duck::invoke_with_zstr(
+		name,
+		m_name_buff.get(),
+		m_name_buff_len,
+		[](const char* name, std::size_t) -> bool {
+			return nullptr == name;
+		}
+	);
 }
 
 auto EnvReal::size() const noexcept -> size_type {