fix: prevent empty .vif files from ec.decode causing parse errors (#7686)

* fix: prevent empty .vif files from ec.decode causing parse errors

When ec.decode copies .vif files from EC shard nodes, if a source node
doesn't have the .vif file, an empty .vif file was created on the target
node. This caused volume.configure.replication to fail with 'proto: syntax
error' when trying to parse the empty file.

This fix:
1. In writeToFile: Remove empty files when no data was written (source
   file was not found) to avoid leaving corrupted empty files
2. In MaybeLoadVolumeInfo: Handle empty .vif files gracefully by treating
   them as non-existent, allowing the system to create a proper one

Fixes #7666

* refactor: remove redundant dst.Close() and add error logging

Address review feedback:
- Remove redundant dst.Close() call since defer already handles it
- Add error logging for os.Remove() failure

weed/server/volume_grpc_copy.go

@@ -291,6 +291,15 @@ func writeToFile(client volume_server_pb.VolumeServer_CopyFileClient, fileName s
 		}
 		wt.MaybeSlowdown(int64(len(resp.FileContent)))
 	}
+	// If no data was written (source file was not found), remove the empty file
+	// to avoid leaving corrupted empty files that cause parse errors later
+	if progressedBytes == 0 && !isAppend {
+		if removeErr := os.Remove(fileName); removeErr != nil {
+			glog.V(1).Infof("failed to remove empty file %s: %v", fileName, removeErr)
+		} else {
+			glog.V(1).Infof("removed empty file %s (source file not found)", fileName)
+		}
+	}
 	return modifiedTsNs, nil
 }