fix: allow to pass null payload to JettonWallet wrapper's sendTransfer and sendBurn

[YakovL]

While transferMessage and burnMessage allow null as payloads, the corresponding send methods do not, hence this fix.

[YakovL]

@tolya-yanot, @EmelyanenkoK, @Pyaepyae-art would you mind merging this? I also have other improvements in mind (like substituting various magic numbers with the corresponding existing constants and more), so let me know if I can expect any collaboration here (I mean review and merge)