Return None if reference_voltage is 0.0

Author: kamtom480

shared-bindings/analogio/AnalogIn.c

@@ -138,7 +138,13 @@ const mp_obj_property_t analogio_analogin_value_obj = {
 STATIC mp_obj_t analogio_analogin_obj_get_reference_voltage(mp_obj_t self_in) {
     analogio_analogin_obj_t *self = MP_OBJ_TO_PTR(self_in);
     check_for_deinit(self);
-    return mp_obj_new_float(common_hal_analogio_analogin_get_reference_voltage(self));
+
+    float reference_voltage = common_hal_analogio_analogin_get_reference_voltage(self);
+    if (reference_voltage == 0.0f) {
+        return mp_const_none; 
+    } else {
+        return mp_obj_new_float(reference_voltage);
+    }
 }
 MP_DEFINE_CONST_FUN_OBJ_1(analogio_analogin_get_reference_voltage_obj,
                           analogio_analogin_obj_get_reference_voltage);