fix the CI risk that network cannot be connected (#22736)

paddle/scripts/paddle_build.sh

@@ -540,6 +540,7 @@ function check_approvals_of_unittest() {
     check_times=$1
     if [ $check_times == 1 ]; then
         approval_line=`curl -H "Authorization: token ${GITHUB_API_TOKEN}" https://api.github.com/repos/PaddlePaddle/Paddle/pulls/${GIT_PR_ID}/reviews?per_page=10000`
+        if [ "${approval_line}" != "" ]; then
             APPROVALS=`echo ${approval_line}|python ${PADDLE_ROOT}/tools/check_pr_approval.py 1 22165420 52485244 6836917`
             set +x
             echo "current pr ${GIT_PR_ID} got approvals: ${APPROVALS}"
@@ -549,6 +550,7 @@ function check_approvals_of_unittest() {
                 echo "==================================="
                 exit 0
             fi
+        fi
     elif [ $check_times == 2 ]; then
         unittest_spec_diff=`python ${PADDLE_ROOT}/tools/diff_unittest.py ${PADDLE_ROOT}/paddle/fluid/UNITTEST_DEV.spec ${PADDLE_ROOT}/paddle/fluid/UNITTEST_PR.spec`
         if [ "$unittest_spec_diff" != "" ]; then