Raise an exception when getting state at an outlier (#12191)

It seems like calling `_get_state_group_for_events` for an event where the
state is unknown is an error. Accordingly, let's raise an exception rather than
silently returning an empty result.

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/12191.misc b/changelog.d/12191.misc
new file mode 100644
index 0000000000..9f333e718a
--- /dev/null
+++ b/changelog.d/12191.misc
@@ -0,0 +1 @@
+Avoid trying to calculate the state at outlier events.