fix: display error message for empty search and search containing whitespace only

src/.vuepress/components/SearchResultEmpty.vue

@@ -2,12 +2,13 @@
   <div>
     <v-alert
       border="left"
-      colored-border
       color="red accent-4"
+      colored-border
       elevation="2"
-    >
+      >
       <!-- TODO: provide translation -->
-      No search results found for <code>{{ query }}</code>
+      <span v-if="validQuery">Please enter a valid search term.</span>
+      <span v-else>No search results found for <code>{{ query }}</code></span>
     </v-alert>
   </div>
 </template>
@@ -20,8 +21,10 @@ export default {
       default: () => '',
     },
   },
+  computed: {
+    validQuery() {
+      return !this.query.match(/\b\s*\b/);
+    },
+  },
 };
 </script>
-
-<style lang="scss" scoped>
-</style>