Hotfix CI determine_tests_to_run (#8039)

ci/travis/ci.sh

@@ -61,7 +61,9 @@ init() {
 
   local variable_definitions
   variable_definitions=($(python "${ROOT_DIR}"/determine_tests_to_run.py))
-  { declare "${variable_definitions[@]}"; } > /dev/null 2> /dev/null
+  if [ 0 -lt "${#variable_definitions[@]}" ]; then
+    declare "${variable_definitions[@]}"
+  fi
 
   if ! (set +x && should_run_job ${job_names//,/ }); then
     exit 0

ci/travis/determine_tests_to_run.py

@@ -56,7 +56,7 @@ if __name__ == "__main__":
             with open(os.environ["GITHUB_EVENT_PATH"], "rb") as f:
                 event = json.loads(f.read())
             base = event["pull_request"]["base"]["sha"]
-            commit_range = "{}...{}".format(base, event["after"])
+            commit_range = "{}...{}".format(base, event.get("after", ""))
         files = list_changed_files(commit_range)
 
         print(pformat(files), file=sys.stderr)