Ability for notes to be sent/consumed without assets

[gubloon]

We started exploring this idea during a discussion with @nlok5923.
Right now, notes are designed to transfer assets between accounts. There can be scenarios where a note can be transferred without assets.
For example, in current use case we want to change state of a game account directly based on the actions of a player account. In such scenario, the player account can send a note to the game account where the game account will authenticate the player account with its signature and updates it state and hence does not have a necessity for the note to store/transfer assets.
Likewise, there can be more similar scenarios where such feature can provide an easy and better way of utilising notes.

[bobbinth]

One of the original reasons for not allowing notes without assets was fees. That is, I assumed that a note would have to carry some assets to pay the fees. But I don't think this is correct any more (the game account could pay fees for the transaction consuming notes). So, yes - we should remove this limitation (and I think it shouldn't be too difficult to do).

[bobbinth]

Closing as this has already been implemented.