Insert into partitioned table fails with NPE if partition column not populated

[martint]

CREATE TABLE t (
    x varchar, 
    y varchar
) 
WITH (partitioned_by = ARRAY['y']);

INSERT INTO t (x) VALUES ('a');

java.lang.NullPointerException
    at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
    at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
    at com.facebook.presto.hive.HivePageSink.createWriter(HivePageSink.java:287)
    at com.facebook.presto.hive.HivePageSink.appendPage(HivePageSink.java:272)
    at com.facebook.presto.spi.classloader.ClassLoaderSafeConnectorPageSink.appendPage(ClassLoaderSafeConnectorPageSink.java:41)
    at com.facebook.presto.operator.TableWriterOperator.addInput(TableWriterOperator.java:176)
    at com.facebook.presto.operator.Driver.processInternal(Driver.java:386)
    at com.facebook.presto.operator.Driver.processFor(Driver.java:303)
    at com.facebook.presto.execution.SqlTaskExecution$DriverSplitRunner.processFor(SqlTaskExecution.java:587)
    at com.facebook.presto.execution.TaskExecutor$PrioritizedSplitRunner.process(TaskExecutor.java:505)
    at com.facebook.presto.execution.TaskExecutor$Runner.run(TaskExecutor.java:640)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

This piece of code in HivePageSink.createWriter() is the culprit:

List<String> partitionValues = partitionRow.stream()
    .map(Object::toString) // todo this seems wrong
    .collect(toList());

[yuananf]

This is the same as #3860