Conditionally registers Agricraft harvest handler.

[Saereth]

Registers the Agricraft harvest handler only if the Agricraft mod is loaded.

Prevents a crash when Agricraft is not present.

[wrincewind]

This does fix the problem in worlds where the crash hasn't yet occured, but when i to fix a world that was already bricked by the issue by updating to this version, it continues to crash with the same error (noClassDefFoundError: com/agricraft/agricraft/common/block/entity/CropBlockEntity').

Do you think you can include a fix to un-brick already broken worlds? if not we can update the changelog to include a note about how to solve this, like 'use the config setting removeErroringBlockEntities in serverconfigs/forge-server.toml', but i'd prefer a more elegant solution if possible.

[Saereth]

I can include a datafixer to sort this yeah, I'll get it in tomorrow since its getting late here today.

[wrincewind]

as per discussion on discord, it'll be safer to keep this commit as-is and include this note in the changelogs:
"in your world save directory, go to /serverconfig/forge-server.toml' and change 'removeErroringBlockEntities' to 'true', launch your world once, then change it back to false."