message: Only set `CULROPT_CAINFO` if the value `ReadStringFromVar` is not-empty (ie, not the
  the default value).
diff: @@ -132,8 +132,10 @@ CurlHttpRequest::CurlHttpRequest(LibCurl* libcurl, Env* env)
  // See also: https://curl.haxx.se/libcurl/c/CURLOPT_CAINFO.html.
  std::string value = "";
  TF_CHECK_OK(ReadStringFromEnvVar("CURL_CA_BUNDLE", "", &value));
  + if (!value.empty()) {
  CHECK_CURL_OK(
  libcurl_->curl_easy_setopt(curl_, CURLOPT_CAINFO, value.c_str()));
  + }
  CHECK_CURL_OK(
  libcurl_->curl_easy_setopt(curl_, CURLOPT_VERBOSE, kVerboseOutput));
  CHECK_CURL_OK(libcurl_->curl_easy_setopt(
  