opt(lb): optimize the performance of weighted_random load balancer

Author: jizhuozhi

volo/src/loadbalance/random.rs

@@ -19,6 +19,9 @@ fn pick_one(
     if sum_of_weight == 0 {
         return None;
     }
+    if instances.len() == 0 {
+        return None;
+    }
     let weight = rand::rng().random_range(0..sum_of_weight);
     let index = prefix_sum_of_weights
         .binary_search(&weight)