Fix issue#6318：该问题导致误判连接池空闲连接不足，创建多余连接

core/src/main/java/com/alibaba/druid/pool/DruidDataSource.java

@@ -3201,7 +3201,9 @@ public class DruidDataSource extends DruidAbstractDataSource
             lock.lock();
             try {
                 int fillCount = minIdle - (activeCount + poolingCount + createTaskCount);
-                emptySignal(fillCount);
+                if (fillCount > 0) {
+                    emptySignal(fillCount);
+                }
             } finally {
                 lock.unlock();
             }