Dashboard width revision message should handle nil (#39710)

* Revision Diff Should handle `nil` Dashboard Width

Even though there shouldn't be `nil` values in dashboard.width, because the migration populates w/ the default value
of `full`, it is still true that because of the version before the migration, revisions  can have `nil` for the width
value. The diff string impl didn't consider this, causing an NPE with `(name nil)`.

* Use :guard to prevent nil values from matching

* Add test checking that dashboard width change is described correctly

src/metabase/models/revision/diff.clj

@@ -89,8 +89,10 @@
     [:result_metadata _ _]
     (deferred-tru "edited the metadata")
 
-    [:width _ _]
-    (deferred-tru "changed the width setting from {0} to {1}" (name v1) (name v2))
+    [:width v1 v2]
+    (if (and v1 v2)
+      (deferred-tru "changed the width setting from {0} to {1}" (name v1) (name v2))
+      (deferred-tru "changed the width setting"))
 
     ;;  whenever database_id, query_type, table_id changed,
     ;; the dataset_query will changed so we don't need a description for this

test/metabase/api/revision_test.clj

@@ -310,6 +310,28 @@
              (map #(select-keys % [:description :has_multiple_changes])
                   (mt/user-http-request :crowberto :get 200 "revision" :entity "dashboard" :id dashboard-id)))))))
 
+(deftest dashboard-width-revision-diff-test
+  (testing "The Dashboard's revision history correctly reports dashboard width changes (#38910)"
+    (t2.with-temp/with-temp
+        [Dashboard  {dashboard-id :id :as dash} {:name "A dashboard"}
+         Revision   _ {:model    "Dashboard"
+                       :model_id dashboard-id
+                       :user_id  (mt/user->id :crowberto)
+                       :object   (assoc dash :width nil)}
+         Revision   _ {:model    "Dashboard"
+                       :model_id dashboard-id
+                       :user_id  (mt/user->id :crowberto)
+                       :object   (assoc dash :width "full")}
+         Revision   _ {:model    "Dashboard"
+                       :model_id dashboard-id
+                       :user_id  (mt/user->id :crowberto)
+                       :object   (assoc dash :width "fixed")}]
+        (is (= ["changed the width setting from full to fixed."
+                "changed the width setting."
+                "modified this."]
+               (map :description
+                    (mt/user-http-request :crowberto :get 200 "revision" :entity "dashboard" :id dashboard-id)))))))
+
 (deftest card-revision-description-test
   (testing "revision description for card are generated correctly"
     (t2.with-temp/with-temp