Eliminate exception for ExportSnapshot against the null table snapshot (with no data in)

In the test environment, when snapshoting the null table (with no data), snapshot artifact can be generated successfully with snapshot meta data, while there is no CF data under region dir. Then when exporting this null snapshot via ExportSnapshot, "java.io.IOException: No input paths specified in job" occurred, which affects the normal application flow.
But seems snapshot for null table also makes sense just with the meta information (.snapshot/), although ".archive/" does not show up. Later snapshot restore seems work from this "dummy" snapshot artifact with no CF data.
Just eliminate exception for ExportSnapshot against the null table snapshot to make application work through.