Table: fix single filter active state

packages/table/src/filter-panel.vue

@@ -19,7 +19,7 @@
     <div class="el-table-filter" v-else v-show="showPopper">
       <ul class="el-table-filter__list">
         <li class="el-table-filter__list-item"
-            :class="{ 'is-active': !filterValue }"
+            :class="{ 'is-active': filterValue === undefined || filterValue === null }"
             @click="handleSelect(null)">{{ t('el.table.clearFilter') }}</li>
         <li class="el-table-filter__list-item"
             v-for="filter in filters"