[index] treat empty string like null when deserializing FileV2 objects

we've seen serialized FileV2 objects becoming an empty string after parcelizing them,
so we need to account for null *and* empty string here.

Fixes acra-crash-reports#617
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f