Fix inconsistencies in event validation for `m.room.create` events (#13087)

* Extend the auth rule checks for `m.room.create` events

... and move them up to the top of the function. Since the no auth_events are
allowed for m.room.create events, we may as well get the m.room.create event
checks out of the way first.

* Add a test for create events with prev_events

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/13087.bugfix b/changelog.d/13087.bugfix
new file mode 100644
index 0000000000..7c69801afe
--- /dev/null
+++ b/changelog.d/13087.bugfix
@@ -0,0 +1 @@
+Fix some inconsistencies in the event authentication code.