camel_data_cache_get: Improve empty file handling.
camel_data_cache_get: Improve empty file handling.

When an empty cache file is detected, set a GError with a descriptive
error message in addition to returning NULL.

tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server