fix failing karma test

app/assets/javascripts/filtered_search/filtered_search_dropdown.js.es6

@@ -4,7 +4,7 @@
   class FilteredSearchDropdown {
     constructor(droplab, dropdown, input, filter) {
       this.droplab = droplab;
-      this.hookId = input.getAttribute('data-id');
+      this.hookId = input && input.getAttribute('data-id');
       this.input = input;
       this.filter = filter;
       this.dropdown = dropdown;

spec/javascripts/filtered_search/dropdown_user_spec.js.es6

@@ -9,7 +9,7 @@ require('~/filtered_search/dropdown_user');
       let dropdownUser;
 
       beforeEach(() => {
-        spyOn(gl.FilteredSearchDropdown.prototype, 'constructor').and.callFake(() => {});
+        spyOn(gl.DropdownUser.prototype, 'bindEvents').and.callFake(() => {});
         spyOn(gl.DropdownUser.prototype, 'getProjectId').and.callFake(() => {});
         spyOn(gl.DropdownUtils, 'getSearchInput').and.callFake(() => {});
 
@@ -39,7 +39,7 @@ require('~/filtered_search/dropdown_user');
 
     describe('config droplabAjaxFilter\'s endpoint', () => {
       beforeEach(() => {
-        spyOn(gl.FilteredSearchDropdown.prototype, 'constructor').and.callFake(() => {});
+        spyOn(gl.DropdownUser.prototype, 'bindEvents').and.callFake(() => {});
         spyOn(gl.DropdownUser.prototype, 'getProjectId').and.callFake(() => {});
       });