IncrementalPartitionedWriter#minCheckpoint isn't safe

[nathankleyn]

The IncrementalPartitionedWriter#minCheckpoint method isn't safe. https://github.com/intenthq/pucket/blob/master/core/src/main/scala/com/intenthq/pucket/writer/IncrementalPartitionedWriter.scala#L42

It tries to call #min on what could potentially be an empty sequence. This should be made safe if possible.