Fix loading config from default paths after merging #199.

This regression was introduced by #199. cfgFiles does point to a slice,
but it was not checked if the slice was empty or not. Therefore, no
configuration file was read if the -c parameter was not used.

Closes chirpstack/chirpstack-docker#74 and
chirpstack/chirpstack-docker#75.

cmd/chirpstack-gateway-bridge/cmd/root.go

@@ -90,7 +90,7 @@ func Execute(v string) {
 }
 
 func initConfig() {
-	if cfgFiles != nil {
+	if cfgFiles != nil && len(*cfgFiles) != 0 {
 		var filesMerged []byte
 		for _, cfgFile := range *cfgFiles {
 			cfgFileContent, err := ioutil.ReadFile(cfgFile)