FIX: Assign default value for category.findByIds method

app/assets/javascripts/discourse/models/category.js.es6

@@ -257,7 +257,7 @@ Category.reopenClass({
     return Category.idMap()[id];
   },
 
-  findByIds(ids) {
+  findByIds(ids = []) {
     const categories = [];
     ids.forEach(id => {
       const found = Category.findById(id);

test/javascripts/models/category-test.js.es6

@@ -196,6 +196,8 @@ QUnit.test("findByIds", assert => {
     Discourse.Category.findByIds([1, 2, 3]),
     _.values(categories)
   );
+
+  assert.deepEqual(Discourse.Category.findByIds(), []);
 });
 
 QUnit.test("search with category name", assert => {