Fix serdes v2 collection load (#29572)

Fixes import of dashboards in the root/default collection

enterprise/backend/test/metabase_enterprise/serialization/v2/e2e_test.clj

@@ -170,8 +170,10 @@
                                                 {:table_id      [:t    10]
                                                  :collection_id [:coll 10]
                                                  :creator_id    [:u    10]}))
-               :dashboard               (many-random-fks 100 {} {:collection_id [:coll 100]
-                                                                 :creator_id    [:u    10]})
+               :dashboard               (concat (many-random-fks 100 {} {:collection_id [:coll 100]
+                                                                         :creator_id    [:u    10]})
+                                                ;; create some root collection dashboards
+                                                (many-random-fks 50 {} {:creator_id    [:u 10]}))
                :dashboard-card          (many-random-fks 300 {} {:card_id      [:c 100]
                                                                  :dashboard_id [:d 100]})
                :dimension               (vec (concat
@@ -270,7 +272,7 @@
                               (reduce +)))))
 
             (testing "for dashboards"
-              (is (= 100 (->> (io/file dump-dir "collections")
+              (is (= 150 (->> (io/file dump-dir "collections")
                               collections
                               (map (comp count dir->file-set #(io/file % "dashboards")))
                               (reduce +)))))

src/metabase/models/dashboard.clj

@@ -495,7 +495,7 @@
   [{:keys [collection_id ordered_cards parameters]}]
   (->> (map serdes-deps-dashcard ordered_cards)
        (reduce set/union)
-       (set/union #{[{:model "Collection" :id collection_id}]})
+       (set/union (when collection_id #{[{:model "Collection" :id collection_id}]}))
        (set/union (serdes/parameters-deps parameters))))
 
 (defmethod serdes/descendants "Dashboard" [_model-name id]