fix(vuepress-theme-reco): replace '??' to '||'

Author: reco_luan

packages/vuepress-theme-reco/components/Mode/ModePicker.vue

@@ -31,8 +31,8 @@ export default {
   mounted () {
     const mode = localStorage.getItem('mode')
     const { mode: customMode, modePicker } = this.$themeConfig
-    const themeMode = customMode ?? 'auto'
-    this.currentMode = modePicker === false ? themeMode : mode ?? themeMode
+    const themeMode = customMode || 'auto'
+    this.currentMode = modePicker === false ? themeMode : mode || themeMode
     setMode(this.currentMode)
   },