rabbit_db_exchange: Relax conditions when creating an exchange with Khepri

[Why]
When the module wanted to create an exchange in Khepri, it used
`rabbit_khepri:create/2` which ensures that the tree node doesn't exist
before. If the tree node exists, it expects it to contain an exchange
record which is returned to the caller, indicating an exchange with that
name already exists.

However, there are several other resources stored under an exchange tree
node, like bindings and topic permissions.

In particular, during a definitions import, topic permissions used to be
imported before exchanges. This caused a crash because the write of the
topic permission automatically created a parent tree node for the
exchange it dpends on, but without an exchange record obviously (see
previous commit).

[How]
As an addition improvement to the previous commit, we change the
conditions: instead of matching on the fact the tree node doesn't exist,
the module also accepts that the tree node exists but has no payload.
Under any of these conditions, the exchange is considered to be new and
written to Khepri.

(cherry picked from commit 5bcbfad)

Author: dumbbell

deps/rabbit/src/rabbit_db_exchange.erl

@@ -409,8 +409,12 @@ create_or_get_in_mnesia(#exchange{name = XName} = X) ->
       end).
 
 create_or_get_in_khepri(#exchange{name = XName} = X) ->
-    Path = khepri_exchange_path(XName),
-    case rabbit_khepri:create(Path, X) of
+    Path0 = khepri_exchange_path(XName),
+    Path1 = khepri_path:combine_with_conditions(
+              Path0, [#if_any{conditions =
+                              [#if_node_exists{exists = false},
+                               #if_has_payload{has_payload = false}]}]),
+    case rabbit_khepri:put(Path1, X) of
         ok ->
             {new, X};
         {error, {khepri, mismatching_node, #{node_props := #{data := ExistingX}}}} ->