public boolean isEmpty() {
    return this.filters.isEmpty();
  }
For each sub-filter in filter list, we save a boolean flag to indicate that whether the return
code of filterCell(c) for sub-filter is INCLUDE* (INCLUDE, INCLUDE_AND_NEXT_COL,
INCLUDE_AND_SEEK_NEXT_ROW) case. if true, we need to transform cell for the sub-filter.
hbase-client/src/main/java/org/apache/hadoop/hbase/filter/FilterListBase.java