Fix compression if zstd is not available
Fix compression if zstd is not available

(cherry picked from commit 56d813b9a3935d2aedb703138fe382a94334ee4e)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend