fix: set assetId to default to undefined instead of 0

[bee344]

Description

As reported on #393, when not specifying the assetId to pay for fees, the value defaulted to something other than undefined as it's supposed to.

Changed the assetId setting in decodeUnsignedTx.ts and defineMethod.ts in order to catch when the assetId is undefined and set it as so.

Previous:

assetId: typeof assetId === 'object'
				? registry.createType('MultiLocation', assetId)
				: registry.createType('Compact<AssetId>', assetId).toNumber()

Current:

assetId: !assetId ? undefined
			: typeof assetId === 'object'
				? registry.createType('MultiLocation', assetId)
				: registry.createType('Compact<AssetId>', assetId).toNumber()

Closes #393