Fix crash on empty write (#930)

libp2p/stream/chronosstream.nim

@@ -129,7 +129,9 @@ method write*(s: ChronosStream, msg: seq[byte]): Future[void] =
   # drives up memory usage
   if msg.len == 0:
     trace "Empty byte seq, nothing to write"
-    return
+    let fut = newFuture[void]("chronosstream.write.empty")
+    fut.complete()
+    return fut
   if s.closed:
     let fut = newFuture[void]("chronosstream.write.closed")
     fut.fail(newLPStreamClosedError())

tests/commontransport.nim

@@ -150,6 +150,7 @@ template commonTransportTest*(prov: TransportProvider, ma1: string, ma2: string
       proc acceptHandler() {.async, gcsafe.} =
         while true:
           let conn = await transport1.accept()
+          await conn.write(newSeq[byte](0))
           await conn.write("Hello!")
           await conn.close()