title: Empty output for Watermarked Aggregation Query in Append Mode
body: <p>I use Spark 2.2.0-rc1.</p>
  
  <p>I've got a Kafka <code>topic</code> which I'm querying a running watermarked aggregation, with a <code>1 minute</code> watermark, giving out to <code>console</code> with <code>append</code> output mode.</p>
  
  <pre><code>import org.apache.spark.sql.types._
  val schema = StructType(StructField("time", TimestampType) :: Nil)
  val q = spark.
    readStream.
    format("kafka").
    option("kafka.bootstrap.servers", "localhost:9092").
    option("startingOffsets", "earliest").
    option("subscribe", "topic").
    load.
    select(from_json(col("value").cast("string"), schema).as("value"))
    select("value.*").
    withWatermark("time", "1 minute").
    groupBy("time").
    count.
    writeStream.
    outputMode("append").
    format("console").
    start
  </code></pre>
  
  <p>I am pushing following data in Kafka <code>topic</code>:</p>
  
  <pre><code>{"time":"2017-06-07 10:01:00.000"}
  {"time":"2017-06-07 10:02:00.000"}
  {"time":"2017-06-07 10:03:00.000"}
  {"time":"2017-06-07 10:04:00.000"}
  {"time":"2017-06-07 10:05:00.000"}
  </code></pre>
  
  <p>And I am getting following output:</p>
  
  <pre class="lang-none prettyprint-override"><code>scala&gt; -------------------------------------------
  Batch: 0
  -------------------------------------------
  +----+-----+                                                                    
  |time|count|
  +----+-----+
  +----+-----+
  
  -------------------------------------------
  Batch: 1
  -------------------------------------------
  +----+-----+                                                                    
  |time|count|
  +----+-----+
  +----+-----+
  
  -------------------------------------------
  Batch: 2
  -------------------------------------------
  +----+-----+                                                                    
  |time|count|
  +----+-----+
  +----+-----+
  
  -------------------------------------------
  Batch: 3
  -------------------------------------------
  +----+-----+                                                                    
  |time|count|
  +----+-----+
  +----+-----+
  
  -------------------------------------------
  Batch: 4
  -------------------------------------------
  +----+-----+                                                                    
  |time|count|
  +----+-----+
  +----+-----+
  </code></pre>
  
  <p>Is this expected behaviour?</p>