Remove redundant null check in ObjectOutputStream.writeProxyDesc

There is implicit null check in line before.

     Class<?>[] ifaces = cl.getInterfaces();
     bout.writeInt(ifaces.length);
     for (int i = 0; i < ifaces.length; i++) {
         bout.writeUTF(ifaces[i].getName());
     }
     bout.setBlockDataMode(true);
     if (cl != null && isCustomSubclass()) {