Don't add empty device to config on init (#8933)

We usually want to ensure that our own device is present. However if the
given device ID is the empty ID, we shouldn't do that. This is a
legimate (though way too non-obvious) use-case when opening the config
without knowing/caring about the device ID.

lib/config/config.go

@@ -268,6 +268,9 @@ func (cfg *Configuration) prepare(myID protocol.DeviceID) error {
 }
 
 func (cfg *Configuration) ensureMyDevice(myID protocol.DeviceID) {
+	if myID == protocol.EmptyDeviceID {
+		return
+	}
 	for _, device := range cfg.Devices {
 		if device.DeviceID == myID {
 			return
@@ -483,6 +486,9 @@ func (cfg *Configuration) SetFolders(folders []FolderConfiguration) {
 }
 
 func ensureDevicePresent(devices []FolderDeviceConfiguration, myID protocol.DeviceID) []FolderDeviceConfiguration {
+	if myID == protocol.EmptyDeviceID {
+		return devices
+	}
 	for _, device := range devices {
 		if device.DeviceID.Equals(myID) {
 			return devices