fix gopath

tool/kratos-protoc/protoc.go

@@ -145,7 +145,7 @@ func latestKratos() (string, error) {
 
 func gopath() (gp string) {
 	gopaths := strings.Split(os.Getenv("GOPATH"), ":")
-	if len(gopaths) == 1 {
+	if len(gopaths) == 1 && gopaths[0] != "" {
 		return gopaths[0]
 	}
 	pwd, err := os.Getwd()
@@ -157,6 +157,9 @@ func gopath() (gp string) {
 		return
 	}
 	for _, gopath := range gopaths {
+		if gopath == "" {
+			continue
+		}
 		absgp, err := filepath.Abs(gopath)
 		if err != nil {
 			return