Handle MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN 'Do nothing for autopilot'

[nsteele]

Describe problem solved by this pull request
I was using MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN to signal an onboard computer to shutdown and for the PX4 autopilot to "do nothing." This is specified by setting param1=0 and param2=2 in the command, but PX4 does not explicitly handle the case of param1=0, which causes PX4 to incorrectly respond with DENIED. My changes cause PX4 to respond with ACCEPTED instead.

Describe your solution
When param1=0, respond with ACCEPTED and do nothing else.

Test data / coverage
The problem is reproducible using a custom QGC widget. I used a widget which includes the following:

QGCButton {
text: "Shutdown Onboard Computer"
// Arguments to CustomCommandWidgetController::sendCommand (MAVLink COMMAND_LONG)
// command id MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN (246) https://mavlink.io/en/messages/common.html#MAV_CMD_PREFLIGHT_REBOOT_SHUTDOWN
// target_component id = 0 (MAV_COMP_ID_ALL)
// confirmation
// param 1 - 7
onClicked: controller.sendCommand(246, 0, 0, 0, 2, 0, 0, 0, 0, 0)
}

Here is a screenshot of what would happen before the changes:

Notice the critical notification near the top of the QGC window. With the code changes, the critical notification no longer appears at the top of the QGC window.