Introduce UdevDeviceGetSysattrValue

device/udev_linux/udev.h

Index: device/udev_linux/udev.h
diff --git a/device/udev_linux/udev.h b/device/udev_linux/udev.h
index d62db6b5aaf329907f08be568191eb7c8c1e42de..c40603d3beafe959583b03c55d228fd3a13a9921 100644
--- a/device/udev_linux/udev.h
+++ b/device/udev_linux/udev.h
@@ -79,6 +79,11 @@ void udev_unref(udev* udev);
 std::string UdevDeviceGetPropertyValue(udev_device* udev_device,
                                        const char* key);
 
+// Calls udev_device_get_sysattr_value() and replaces missing values with
+// the empty string.
+std::string UdevDeviceGetSysattrValue(udev_device* udev_device,
+                                      const char* key);
+
 // Decodes udev-encoded string. Useful for decoding "*_ENC" udev properties.
 std::string UdevDecodeString(const std::string& encoded);