Starting from 9.0.2, contactMergeDataUpgrade.log.{date}.txt log files are created after each Sitecore start. The files are populated during running the \sitecore\admin\xdb\ContactsMerge.aspx page, which is required during upgrade from 9.0.0 and 9.0.1 to 9.0.2 and later (details can be found in the corresponding Upgrade Guide). If the page is not run, the files remain empty. This behavior is expected because Sitecore XP creates new log files per log4net appender configured even if there is no data flushed.
You can safely ignore such behavior. Otherwise, if you want to prevent empty log files from being created, refer to the solution below.