reissuance tokens can not be exercised from unblinded outputs

In the current logic, `CAssetIssuance`'s `assetBlindingNonce` being null implies that the issuance is an initial issuance. When re-issuing, this field is re-used to expose the asset blinder for the spent output.

Clearly this is in direct conflict when the output itself has a null blinder.

If the user attempts to make a reissuance with the null blinder output, it will be interpreted as an initial issuance, using the reissuance token purely as the entropy.