Handle file group building for log file only case

[xushiyan]

When doing upsert to MOR tables with bucket index from the beginning, before the first compaction runs, file groups contain only log files. This can should be handled properly. Search for related TODO comment: /// TODO: support adding log files to file group without base files.

[TheR1sing3un]

@xushiyan Hi, shiyan, I would like to fix it, please assign it to me~