## What changes were proposed in this pull request?
When a streaming query has multiple file streams, and there is a batch where one of the file streams dont have data in that batch, then if the query has to restart from that, it will throw the following error.
```
java.lang.IllegalStateException: batch 1 doesn't exist
	at org.apache.spark.sql.execution.streaming.HDFSMetadataLog$.verifyBatchIds(HDFSMetadataLog.scala:300)
	at org.apache.spark.sql.execution.streaming.FileStreamSourceLog.get(FileStreamSourceLog.scala:120)
	at org.apache.spark.sql.execution.streaming.FileStreamSource.getBatch(FileStreamSource.scala:181)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$org$apache$spark$sql$execution$streaming$MicroBatchExecution$$populateStartOffsets$2.apply(MicroBatchExecution.scala:294)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$org$apache$spark$sql$execution$streaming$MicroBatchExecution$$populateStartOffsets$2.apply(MicroBatchExecution.scala:291)
	at scala.collection.Iterator$class.foreach(Iterator.scala:891)
	at scala.collection.AbstractIterator.foreach(Iterator.scala:1334)
	at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
	at org.apache.spark.sql.execution.streaming.StreamProgress.foreach(StreamProgress.scala:25)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution.org$apache$spark$sql$execution$streaming$MicroBatchExecution$$populateStartOffsets(MicroBatchExecution.scala:291)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$runActivatedStream$1$$anonfun$apply$mcZ$sp$1.apply$mcV$sp(MicroBatchExecution.scala:178)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$runActivatedStream$1$$anonfun$apply$mcZ$sp$1.apply(MicroBatchExecution.scala:175)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$runActivatedStream$1$$anonfun$apply$mcZ$sp$1.apply(MicroBatchExecution.scala:175)
	at org.apache.spark.sql.execution.streaming.ProgressReporter$class.reportTimeTaken(ProgressReporter.scala:251)
	at org.apache.spark.sql.execution.streaming.StreamExecution.reportTimeTaken(StreamExecution.scala:61)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution$$anonfun$runActivatedStream$1.apply$mcZ$sp(MicroBatchExecution.scala:175)
	at org.apache.spark.sql.execution.streaming.ProcessingTimeExecutor.execute(TriggerExecutor.scala:56)
	at org.apache.spark.sql.execution.streaming.MicroBatchExecution.runActivatedStream(MicroBatchExecution.scala:169)
	at org.apache.spark.sql.execution.streaming.StreamExecution.org$apache$spark$sql$execution$streaming$StreamExecution$$runStream(StreamExecution.scala:295)
	at org.apache.spark.sql.execution.streaming.StreamExecution$$anon$1.run(StreamExecution.scala:205)
```

Existing `HDFSMetadata.verifyBatchIds` threw error whenever the `batchIds` list was empty. In the context of `FileStreamSource.getBatch` (where verify is called) and `FileStreamSourceLog` (subclass of `HDFSMetadata`), this is usually okay because, in a streaming query with one file stream, the `batchIds` can never be empty:
- A batch is planned only when the `FileStreamSourceLog` has seen new offset (that is, there are new data files).
- So `FileStreamSource.getBatch` will be called on X to Y where X will always be > Y. This calls internally`HDFSMetadata.verifyBatchIds (X+1, Y)` with X+1-Y ids.

For example.,`FileStreamSource.getBatch(4, 5)` will call `verify(batchIds = Seq(5), start = 5, end = 5)`. However, the invariant of X > Y is not true when there are two file stream sources, as a batch may be planned even when only one of the file streams has data. So one of the file stream may not have data, which can call `FileStreamSource.getBatch(X, X)` -> `verify(batchIds = Seq.empty, start = X+1, end = X)` -> failure.

Note that `FileStreamSource.getBatch(X, X)` gets called **only when restarting a query in a batch where a file source did not have data**. This is because in normal planning of batches, `MicroBatchExecution` avoids calling `FileStreamSource.getBatch(X, X)` when offset X has not changed. However, when restarting a stream at such a batch, `MicroBatchExecution.populateStartOffsets()` calls `FileStreamSource.getBatch(X, X)` (DataSource V1 hack to initialize the source with last known offsets) thus hitting this issue.

The minimum solution here is to skip verification when `FileStreamSource.getBatch(X, X)`.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)
(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

Please review http://spark.apache.org/contributing.html before opening a pull request.

Closes #23557 from tdas/SPARK-26629.

Authored-by: Tathagata Das <tathagata.das1565@gmail.com>
Signed-off-by: Shixiong Zhu <zsxwing@gmail.com>