HDFS-17461. Modify patch based on comments

Author: haiyang1987

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/PeerCache.java

@@ -155,10 +155,6 @@ public Peer get(DatanodeID dnId, boolean isDomain) {
 
   private synchronized Peer getInternal(DatanodeID dnId, boolean isDomain) {
     List<Value> sockStreamList = multimap.get(new Key(dnId, isDomain));
-    if (sockStreamList.isEmpty()) {
-      return null;
-    }
-
     Iterator<Value> iter = sockStreamList.iterator();
     while (iter.hasNext()) {
       Value candidate = iter.next();