Fix tests for frontend installation

lib/pleroma/web/api_spec/operations/admin/frontend_operation.ex

@@ -51,7 +51,7 @@ defp list_of_frontends do
           name: %Schema{type: :string},
           git: %Schema{type: :string, format: :uri, nullable: true},
           build_url: %Schema{type: :string, format: :uri, nullable: true},
-          ref: %Schema{type: :string},
+          ref: %Schema{type: :string, nullable: true},
           installed: %Schema{type: :boolean},
           installed_refs: %Schema{type: :array, items: %Schema{type: :string}}
         }

test/pleroma/web/admin_api/controllers/frontend_controller_test.exs

@@ -89,6 +89,7 @@ test "from available frontends", %{conn: conn} do
                  "build_url" => "http://gensokyo.2hu/builds/${ref}",
                  "git" => nil,
                  "installed" => true,
+                 "installed_refs" => ["fantasy"],
                  "name" => "pleroma",
                  "ref" => "fantasy"
                }