Slider: mitigate high cpu usage (#8989)

see: https://github.com/ElemeFE/element/issues/8989

packages/slider/src/main.vue

@@ -260,6 +260,7 @@
 
     computed: {
       stops() {
+        if (!this.showStops) return [];
         if (this.step === 0) {
           process.env.NODE_ENV !== 'production' &&
           console.warn('[Element Warn][Slider]step should not be 0.');