HAL_Linux: use nullptr for vnav rcinput

this makes it clearer what is going on

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -164,8 +164,9 @@ static RCInput_Navio2 rcinDriver;
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_RST_ZYNQ
 static RCInput_RCProtocol rcinDriver{"/dev/ttyPS0", NULL};
 #elif CONFIG_HAL_BOARD_SUBTYPE == HAL_BOARD_SUBTYPE_LINUX_VNAV
-// this is needed to allow for RC input using SERIALn_PROTOCOL=23
+// this is needed to allow for RC input using SERIALn_PROTOCOL=23. No fd is opened
-static RCInput_RCProtocol rcinDriver{"/dev/null", NULL};
+// in the linux driver and instead user needs to provide a uart via SERIALn_PROTOCOL
+static RCInput_RCProtocol rcinDriver{nullptr, nullptr};
 #else
 static RCInput rcinDriver;
 #endif