fix(14-to-15): ignore Bootstrap empty

[Dreamacro]

if kubo startup with ipfs bootstrap rm all, the bootstrap list is empty.

current output

applying 14-to-15 repo migration
> Upgrading config to new format
error: invalid type for .Bootstrap got <nil> expected json array

[gammazero]

@lidel Do we want to bother making this migration, or should we just close this with with a known work-around.

[lidel]

I think this ship sailed, we are closing this because we don't want to re-release penultimate migration.