set best sensor to -1 after last sensor fails (#23425)

src/modules/sensors/data_validator/DataValidatorGroup.cpp

@@ -230,6 +230,10 @@ float *DataValidatorGroup::get_best(uint64_t timestamp, int *index)
 				if (_first_failover_time == 0) {
 					_first_failover_time = timestamp;
 				}
+
+				if (max_confidence < FLT_EPSILON) {
+					max_index = -1;
+				}
 			}
 		}