fix(github): catch empty graphql data

lib/platform/github/index.js

@@ -890,6 +890,11 @@ async function getOpenPrs() {
       };
       const res = JSON.parse((await get.post(url, options)).body);
       const prNumbers = [];
+      // istanbul ignore if
+      if (!res.data) {
+        logger.warn({ res }, 'No graphql res.data');
+        return {};
+      }
       for (const pr of res.data.repository.pullRequests.nodes) {
         // https://developer.github.com/v4/object/pullrequest/
         pr.displayNumber = `Pull Request #${pr.number}`;
@@ -986,6 +991,11 @@ async function getClosedPrs() {
       };
       const res = JSON.parse((await get.post(url, options)).body);
       const prNumbers = [];
+      // istanbul ignore if
+      if (!res.data) {
+        logger.warn({ res }, 'No graphql res.data');
+        return {};
+      }
       for (const pr of res.data.repository.pullRequests.nodes) {
         // https://developer.github.com/v4/object/pullrequest/
         pr.displayNumber = `Pull Request #${pr.number}`;