Change: Empty files now have size of 0 in `ls --json` output

The `ls --json` command used to omit the sizes of empty files in its
output. It now reports a size of zero explicitly for regular files,
while omitting the size field for all other types.

https://github.com/restic/restic/issues/3247
https://github.com/restic/restic/pull/3257
