Set the right isolation level in the commit info

src/main/scala/org/apache/spark/sql/delta/OptimisticTransaction.scala

@@ -380,7 +380,7 @@ trait OptimisticTransactionImpl extends TransactionalWrite with SQLMetricsReport
           op.jsonEncodedValues,
           Map.empty,
           Some(readVersion).filter(_ >= 0),
-          None,
+          Option(isolationLevelToUse.toString),
           Some(isBlindAppend),
           getOperationMetrics(op),
           getUserMetadata(op))
@@ -623,7 +623,8 @@ trait OptimisticTransactionImpl extends TransactionalWrite with SQLMetricsReport
       numFilesTotal = postCommitSnapshot.numOfFiles,
       sizeInBytesTotal = postCommitSnapshot.sizeInBytes,
       protocol = postCommitSnapshot.protocol,
-      info = Option(commitInfo).map(_.copy(readVersion = None, isolationLevel = None)).orNull,
+      info = Option(commitInfo)
+        .map(_.copy(readVersion = None, isolationLevel = Option(isolationLevel.toString))).orNull,
       newMetadata = newMetadata,
       numAbsolutePaths,
       numDistinctPartitionsInAdd = distinctPartitions.size,

src/test/scala/org/apache/spark/sql/delta/OptimisticTransactionSuite.scala

@@ -737,4 +737,18 @@ class OptimisticTransactionSuite extends QueryTest with SharedSparkSession {
       }
     }
   }
+
+  test("isolation level shouldn't be null") {
+    withTempDir { tempDir =>
+      val log = DeltaLog(spark, new Path(tempDir.getCanonicalPath))
+      // Initialize the log.
+      log.startTransaction().commitManually()
+
+      val txn = log.startTransaction()
+      txn.commit(addA :: Nil, ManualUpdate)
+
+      val isolationLevels = log.history.getHistory(Some(10)).map(_.isolationLevel)
+      assert(isolationLevels === Seq(Option("Serializable"), Option("SnapshotIsolation")))
+    }
+  }
 }