Fix multiple coder bug in new spark runner

The test below does not pass. I fails with an EOF while calling NullableCoder(BigEndianCoder.decode) 

'org.apache.beam.sdk.transforms.FlattenTest.testFlattenMultipleCoders'

In the "old" spark runner, this test passes because it never call the NullableCoder because there is no serialization done. 

There may be a problem in the NullableCoder itself