message: builder: fix panic on merge
  Fix panic on nydus-image merge command:
  ```
  thread 'main' panicked at 'called `Option::unwrap()` on a `None` value': src/bin/nydus-image/merge.rs:183
  ```
  The fs_version should be set in previous code.
diff: @@ -72,15 +72,14 @@ impl Merger {
  let rs = RafsSuper::load_from_metadata(bootstrap_path, RafsMode::Direct, true)
  .context(format!("load bootstrap {:?}", bootstrap_path))?;
  - if let Some(config) = &config {
  - config.check_compatibility(&rs.meta)?;
  - } else {
  - config = Some(rs.meta.get_config());
  + config
  + .get_or_insert_with(|| rs.meta.get_config())
  + .check_compatibility(&rs.meta)?;
  +
  fs_version = Some(rs.meta.version);
  ctx.compressor = rs.meta.get_compressor();
  ctx.digester = rs.meta.get_digester();
  ctx.explicit_uidgid = rs.meta.explicit_uidgid();
  - }
  let blob_hash = Self::get_blob_hash(bootstrap_path)?;
  let mut blob_idx_map = Vec::new();
  