The "empty index" option on the main wiki Administration: Search does not work for a list of wikis

Steps to reproduce:

• go on the main wiki Administration: Search
• on the Custom index section Wikis add a list of wikis from the farm
• tick "Clear the index"
• click "start indexing"

Result:

• the reindex does not clean the previous index

  ...
   if (this.rebuildInProgress) {
              LOGGER.warn("Cannot launch rebuild because another rebuild is in progress");
  
              return LucenePluginApi.REBUILD_IN_PROGRESS;
          } else {
              if (clearIndex) {
                  if (wikis == null) {
                      this.indexUpdater.cleanIndex();
                  } else {
                      // TODO: clean wikis listed in wikis
                  }
              }
  ...
  

  This feature does not seem to be implemented on https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-lucene/src/main/java/com/xpn/xwiki/plugin/lucene/IndexRebuilder.java