Prepare action callback should support meta without payload

[markerikson]

Per #148 (comment) , it looks like you can't have meta without also having payload. We should allow both or either individually.

[markerikson]

Mmm... I'm questioning whether this is worth the effort.

The simple workaround is to have a prepare callback like () => ({payload: undefined, meta : someValue}).

I don't think it's worth complicating the types any further just to support this.

Closing for now.

( @phryneas , @Jessidhia, feel free to chime in if you feel this is relatively easily doable.)