Allow detection of issues that have an empty stacktrace
I noticed that in some cases, failures don't have a stacktrace (https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/731#note_461793083). In those cases, we currently create duplicate issues (e.g. https://gitlab.com/rymai/gitlab-qa-issues/-/issues/77 and https://gitlab.com/rymai/gitlab-qa-issues/-/issues/78).
This MR fixes that so that issues without a stacktrace can still be detected as duplicates.
Edited by George Koltsov