HBASE-26688 Threads shared EMPTY_RESULT may lead to unexpected client job down. (#4048)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

Author: YutSean

hbase-client/src/main/java/org/apache/hadoop/hbase/client/Result.java

@@ -913,16 +913,19 @@ public CellScanner cellScanner() {
 
   @Override
   public Cell current() {
-    if (cells == null
+    if (isEmpty()
             || cellScannerIndex == INITIAL_CELLSCANNER_INDEX
-            || cellScannerIndex >= cells.length)
+            || cellScannerIndex >= cells.length) {
       return null;
+    }
     return this.cells[cellScannerIndex];
   }
 
   @Override
   public boolean advance() {
-    if (cells == null) return false;
+    if (isEmpty()) {
+      return false;
+    }
     cellScannerIndex++;
     if (cellScannerIndex < this.cells.length) {
       return true;