Fix CSV export with MongoDB transient store

Steps to reproduce:

1. install 2021 HF30
2. configure Nuxeo to use MongoDB and add this line to nuxeo.conf

   nuxeo.transientstore.provider=mongodb

3. start Nuxeo
4. navigate to the domain
5. click on the CSV Export button
6. observe that an .html file is downloaded and with Chrome, you should see the message "Failed - Server problem" in the status bar
7. In the logs, there is this stacktrace

   Caused by: java.lang.NullPointerException
           at org.nuxeo.ecm.core.mongodb.transientstore.MongoDBTransientStore.getParameter(MongoDBTransientStore.java:331) ~[nuxeo-core-mongodb-2021.30.14.jar:?]
           at org.nuxeo.ecm.core.io.download.DownloadServiceImpl.downloadBlob(DownloadServiceImpl.java:434) ~[nuxeo-core-io-2021.29.7.jar:?]
           at org.nuxeo.ecm.core.io.download.DownloadServiceImpl.downloadBlob(DownloadServiceImpl.java:416) ~[nuxeo-core-io-2021.29.7.jar:?]
           at org.nuxeo.ecm.core.io.download.DownloadServiceImpl.handleDownload(DownloadServiceImpl.java:322) ~[nuxeo-core-io-2021.29.7.jar:?]
           at org.nuxeo.ecm.platform.ui.web.download.DownloadServlet.handleDownload(DownloadServlet.java:83) ~[nuxeo-platform-web-common-2021.27.13.jar:?]
           at org.nuxeo.ecm.platform.ui.web.download.DownloadServlet.doGet(DownloadServlet.java:61) ~[nuxeo-platform-web-common-2021.27.13.jar:?]
   

Expected behavior: the CSV export is downloaded without error