[FIX] Yggdrasil return have dust left in the yggdrasil wallet , which will stop node operator from unbond

On ETH chain , given we transfer both ETH.ETH and ERC20 tokens using Router smart contract , with that , it is really hard to drain the wallet entirely , it always have a little bit dust left over in the wallet If the node operator would like to leave, then they will be slashed with the dust If the node operator would like to stay and unbond some profit , they won't be able to do that , as yggdrasil vault still have fund in it.

It can be fixed manually by send in some more fund and send it all to asgard , but that is manual

We should add logic in place , if the amount of asset in yggdrasil wallet is only gas asset , and it is not enough to pay for gas , then unbond should be allowed.