config: fix in memory config not saving on the fly backend config

Before this fix, saving a :backend config gave the error

    Can't save config "token" = "XXX" for on the fly backend ":backend"

Even when using the in-memory config `--config ""`

This fixes the problem by
- always using the in memory config if it is configured
- moving the check for a :backend config save to the file config backend

It also removes the contents of the config items being saved from the
log which saves confidential tokens being logged.

Fixes #5451

fs/config/config.go

@@ -332,6 +332,10 @@ func SetConfigPath(path string) (err error) {
 
 // SetData sets new config file storage
 func SetData(newData Storage) {
+	// If no config file, use in-memory config (which is the default)
+	if configPath == "" {
+		return
+	}
 	data = newData
 	dataLoaded = false
 }
@@ -371,10 +375,6 @@ var ErrorConfigFileNotFound = errors.New("config file not found")
 // SaveConfig calling function which saves configuration file.
 // if SaveConfig returns error trying again after sleep.
 func SaveConfig() {
-	if configPath == "" {
-		fs.Debugf(nil, "Skipping save for memory-only config")
-		return
-	}
 	ctx := context.Background()
 	ci := fs.GetConfig(ctx)
 	var err error