[jvm-packages] fix "key not found: train" issue (#6842)

* [jvm-packages] fix "key not found: train" issue

* fix bug

jvm-packages/xgboost4j-spark/src/main/scala/ml/dmlc/xgboost4j/scala/spark/XGBoost.scala

@@ -377,7 +377,7 @@ object XGBoost extends Serializable {
     // to workaround the empty partitions in training dataset,
     // this might not be the best efficient implementation, see
     // (https://github.com/dmlc/xgboost/issues/1277)
-    if (watches.toMap("train").rowNum == 0) {
+    if (!watches.toMap.contains("train")) {
       throw new XGBoostError(
         s"detected an empty partition in the training data, partition ID:" +
           s" ${TaskContext.getPartitionId()}")