CN-1377 国际化内容补充:1.检查下拉框，去掉placeholder;

src/views/charts2/charts/dnsInsight/DnsTrafficLine.vue

@@ -33,6 +33,7 @@
             <el-select
                 size="mini"
                 v-model="lineMetric"
+                placeholder=" "
                 popper-class="common-select"
                 :popper-append-to-body="false"
                 @change="metricSelectChange"
@@ -47,6 +48,7 @@
             <el-select
                 size="mini"
                 v-model="lineRefer"
+                placeholder=" "
                 :disabled="!lineTab"
                 popper-class="common-select"
                 :popper-append-to-body="false"

src/views/charts2/charts/entityDetail/EntityDetailLine.vue

@@ -66,6 +66,7 @@
                   size="mini"
                   v-model="lineRefer"
                   :disabled="!lineTab"
+                  placeholder=" "
                   popper-class="common-select"
                   :popper-append-to-body="false"
                   @change="referenceSelectChange"

src/views/charts2/charts/linkMonitor/LinkTrafficLine.vue

@@ -42,6 +42,7 @@
              <el-select
                  size="mini"
                  v-model="lineMetric"
+                 placeholder=" "
                  popper-class="common-select"
                  :popper-append-to-body="false"
                  @change="metricSelectChange"

src/views/charts2/charts/networkOverview/NetworkOverviewLine.vue

@@ -38,6 +38,7 @@
                 size="mini"
                 v-model="lineRefer"
                 :disabled="!lineTab"
+                placeholder=" "
                 popper-class="common-select"
                 :popper-append-to-body="false"
                 @change="referenceSelectChange"

src/views/charts2/charts/npm/NpmMap.vue

@@ -9,6 +9,7 @@
           v-model="trafficDirection"
           class="map-select map-select__direction"
           popper-class="map-select-down"
+          placeholder=" "
           :popper-append-to-body="false"
       >
         <el-option value="Server">Server</el-option>