public static Command getCommand() {
    return singleton;
  }
All keys of the register interest list are being sent as a single part since 6.6. There is no
need to send no keys as a separate part. In earlier versions the number
of keys & each individual key was sent as a separate part.

@since GemFire 6.6
geode-core/src/main/java/org/apache/geode/internal/cache/tier/sockets/command/RegisterInterestList66.java