Apply frontend review suggestions

- Clarity the intent that fullpath should only be passed
for generating iteration reports.

- Update spec description

ee/app/assets/javascripts/burndown_chart/components/burn_charts.vue

@@ -78,7 +78,7 @@ export default {
       },
       query: BurnupQuery,
       variables() {
-        const fullPath = this.iterationId ? { fullPath: this.fullPath } : {};
+        const fullPath = this.isIterationReport ? { fullPath: this.fullPath } : {};
 
         return {
           id: this.iterationId || this.milestoneId,
@@ -129,6 +129,9 @@ export default {
     burnupData() {
       return this.report.burnupData;
     },
+    isIterationReport() {
+      return this.iterationId && !this.milestoneId;
+    },
     columns() {
       return [
         {

ee/spec/frontend/burndown_chart/components/burn_charts_spec.js

@@ -287,7 +287,7 @@ describe('burndown_chart', () => {
     });
   });
 
-  describe('with Apollo mock', () => {
+  describe('fullPath is only passed for iteration report', () => {
     let burnupQuerySpy;
 
     const createComponentWithApollo = ({ props = {} }) => {