save(true) not work when getKey is empty

[amirrezasalimi]

hi see my codes when cache is empty getKey return undefined and i cant use push over that to save with prune

but this worked when add this code to cache.js https://github.com/royriojas/flat-cache/blob/9e95cdca746ccc5c066d277f1edde9c9b9f95348/cache.js#L80

please fix thanks