Fix manager panic

[chris124567]

Fix case where chain.Manager would panic when adding a v1 transaction after the v2 hardfork require height.

[lukechampine]

I decided to move the check into the Store; I think it's a slight improvement to say "the supplement for a v2 block is defined as empty," rather than checking the height wherever we might need to compute a supplement. ‾\_(ツ)_/‾