fetch_gaidao.sh: Do not try to download nothingness.

fetch_gaidao.sh

@@ -9,7 +9,7 @@ for ausgabe in ${ausgaben}; do
     url=$(curl -s "${ausgabe}" \
               | grep -Eo 'https://fda-ifa.org/wp-content/uploads/[0-9]{4}/[0-9]{2}/[^ ]*\.epub')
     file=$(echo "${url}" | grep -o '[^/]*\.epub')
-    if [ ! -e "${file}" ]; then
+    if [ -n "${file}" ] && [ ! -e "${file}" ]; then
         echo "Downloading Gai Dao: ${file}."
         curl -so "${file}" "${url}"
     fi