backport: fix empty path segment in top containers search

The staff UI's "top containers" search page is not working after configuring Jetty to block ambiguous URLs, because the page makes API requests to the backend with a URL path containing //. ArchivesSpace also bumped into this when they upgraded to Jetty 10 in 4.0, so to fix this we're backporting the upstream fix for the same issue.

This fixes https://cambridge-collections.atlassian.net/browse/AR-405.