SCollection#unionAll throws an exception on empty list #1092
Description
When passed an empty Iterable[SCollection[T]], SCollection#unionAll throws an exception:
Exception in thread "main" java.lang.IllegalArgumentException: must either have a non-empty list of PCollections, or must first call empty(Pipeline)
at org.apache.beam.sdk.values.PCollectionList.of(PCollectionList.java:97)
at com.spotify.scio.values.SCollection$.unionAll(SCollection.scala:71)
Scio should probably handle this case properly before calling into PCollectionList, as the union of an empty list of collections is logically an empty collection.