Fix bug with empty consumer buffers which mostly affected Linux

Sources/GzipMiddleware/GzipMiddleware.swift

@@ -44,6 +44,7 @@ public struct GzipServerMiddleware: Middleware, ServiceType {
                         return data.subdata(in: offset..<offset+readSize)
                     },
                     consumer: { data -> Void in
+		    	guard data.count > 0 else { return } // Skip empty buffers
                         var buffer  = ByteBufferAllocator().buffer(capacity: bufSize)
                         buffer.write(bytes: data)
                         write = write.flatMap {