rewritemeta: do not include empty Summary: or Description:

Since the Summary: and Description: in the metadata file has the highest
priority of all the localized texts, adding blank versions means that
apps would always have blank Summary and Description even if the app has
those fields in the localized sections of fdroiddata and/or in the app's
source repo itself.

fdroiddata!2262
2 jobs for fdroiddata-localization-fixes in 26 minutes and 14 seconds
Status Job ID Name Coverage
  Test
passed #18135004
metadata_v0

00:03:37

passed #18135003
test

00:26:14