check for empty PST

and return appropriate returnvalue

src/fsfw/returnvalues/FwClassIds.h

@@ -72,10 +72,12 @@ enum: uint8_t {
     PUS_SERVICE_3, //PUS3
     PUS_SERVICE_9, //PUS9
     FILE_SYSTEM, //FILS
+    LINUX_OSAL, //UXOS
     HAL_SPI, //HSPI
     HAL_UART, //HURT
     HAL_I2C, //HI2C
     HAL_GPIO, //HGIO
+    FIXED_SLOT_TASK_IF, //FTIF
     FW_CLASS_ID_COUNT // [EXPORT] : [END]
 
 };