lua-factory: Add comment in grl_util_build_media()

For the next person that will try to unfold the switch statement.
lua-factory: Don't warn for empty values

Sometimes, values will be set to nil for certain keys, which should
simply be ignored. Don't print warnings or debug in those cases.
lua-factory: Remove confusing fetch_op_cleanup

fetch_op_cleanup didn't really cleanup the whole operation structure.
Instead, clean up what we need to and wait for the rest of the
operations to complete.
Updated Slovenian translation