MockedStreams.output shouldn't raise ExpectedOutputIsEmpty

[eliaslevy]

MockedStreams.output doesn't allow you to specify zero for the size of the expected output. That means you can't set an expectation that certain inputs should not have any outputs in your topology.

Instead of raising an exception when given a size of zero, output should verify that driver.readOutput returns null and return an empty set.

[spacer59]

maybe relevant but not defining output (even for terminating streams) does not materialize the stream, and as a result verifications fail.