refactor: fix check for empty string

It is not recommended to use `len` for empty string test.

api/middleware.go

@@ -18,7 +18,7 @@ const (
 func authMiddleware(tokenMaker token.Maker) gin.HandlerFunc {
 	return func(ctx *gin.Context) {
 		authHeader := ctx.GetHeader(authHeaderKey)
-		if len(authHeader) == 0 {
+		if authHeader == "" {
 			err := errors.New("authorization header is not provided")
 			ctx.AbortWithStatusJSON(http.StatusUnauthorized, errorResponse(err))
 			return