Tweak csv_export_modal.vue props

Make props required since they're being defaulted in
csv_import_export_buttons.vue

app/assets/javascripts/issuable/components/csv_export_modal.vue

@@ -21,13 +21,11 @@ export default {
   props: {
     exportCsvPath: {
       type: String,
-      required: false,
-      default: '',
+      required: true,
     },
     issuableCount: {
       type: Number,
-      required: false,
-      default: 0,
+      required: true,
     },
     modalId: {
       type: String,

spec/frontend/issuable/components/csv_export_modal_spec.js

@@ -13,6 +13,8 @@ describe('CsvExportModal', () => {
       mount(CsvExportModal, {
         propsData: {
           modalId: 'csv-export-modal',
+          exportCsvPath: 'export/csv/path',
+          issuableCount: 1,
           ...props,
         },
         provide: {