Bugfix for alter consumers in pqrb (ydb-platform#12613)

Author: FloatingCrowbar

ydb/core/persqueue/read_balancer.cpp

@@ -527,6 +527,8 @@ void TPersQueueReadBalancer::Handle(TEvPersQueue::TEvUpdateBalancerConfig::TPtr
         auto it = oldConsumers.find(consumer.GetName());
         if (it != oldConsumers.end()) {
             Consumers[consumer.GetName()] = std::move(it->second);
+        } else {
+            Consumers.insert(std::make_pair(consumer.GetName(), TConsumerInfo{}));
         }
     }