Update experiments enrollments warnings to account for experiments with no end date

[scholtzan]

Follow-up to https://github.com/mozilla/experimenter/issues/7176

The experiments enrollments dashboard shows a warning if the unenrollments are elevated. However, for experiments that do not have and end date set, this check is ignored.

back when when we setup jetstream, we decided to have endDate be null until the experiment ends which is the signal jetstream uses to determine which experiments to run analysis for and also becuase if it's filled in with the projected end date, and then the experiment ends some other time and that date changes, it would be difficult for jetstream to have to keep track and compare the diff

We should update the dashboard to account for this.

┆Issue is synchronized with this Jira Task