SelectionMixin.js: fix whitespace to eslint settings

cateditor/src/components/SelectionMixin.js

@@ -66,7 +66,7 @@ const SelectionMixin = { // eslint-disable-line no-unused-vars
     sectionIDs: function(section) {
       let ids = []
 
-      if (section !== undefined) {
+      if (section) {
         for (let page of section.pages) {
           for (let block of page) {
             ids.push(...block.ids)