RATIS-1991. Optimize the reconfig judgment logic in the ReconfigurationBase class. (#1004)

diff --git a/ratis-common/src/main/java/org/apache/ratis/conf/ReconfigurationBase.java b/ratis-common/src/main/java/org/apache/ratis/conf/ReconfigurationBase.java
index 6d4930e..8123e24 100644
--- a/ratis-common/src/main/java/org/apache/ratis/conf/ReconfigurationBase.java
+++ b/ratis-common/src/main/java/org/apache/ratis/conf/ReconfigurationBase.java
@@ -172,7 +172,7 @@
     }
     final String effective = reconfigureProperty(property, newValue);
     LOG.info("{}: changed property {} to {} (effective {})", name, property, newValue, effective);
-    if (newValue != null) {
+    if (effective != null) {
       properties.set(property, effective);
     } else {
       properties.unset(property);