CloseGateOutputSignal on empty buffer shouldn't throw an exception

[asubb]

Sometimes, you may need to avoid starting writing to the file at the very beginning and send the CloseGateOutputSignal with the first sample. Currently it is unsupported, throws something like:

java.lang.IllegalStateException: [io.wavebeans.lib.io.FileWriterDelegate@fff7bc7] Attempting to finalize uninitialized buffer
	at io.wavebeans.lib.io.FileWriterDelegate.doFinalizeBuffer(FileWriterDelegate.kt:108)
	at io.wavebeans.lib.io.FileWriterDelegate.finalizeBuffer(FileWriterDelegate.kt:96)
	at io.wavebeans.lib.io.AbstractPartialWriter.write(AbstractWriter.kt:136)
	at io.wavebeans.execution.SingleThreadedOverseer$eval$$inlined$map$lambda$1.call(SingleThreadedOverseer.kt:35)
	at io.wavebeans.execution.SingleThreadedOverseer$eval$$inlined$map$lambda$1.call(SingleThreadedOverseer.kt:15)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)