Fixed the API endpoint /api/tags when the model list is empty. (#4424)

* Fixed the API endpoint /api/tags to return {models: []} instead of {models: null} when the model list is empty.

* Update server/routes.go

---------

Co-authored-by: Jeffrey Morgan <jmorganca@gmail.com>

server/routes_test.go

@@ -95,6 +95,7 @@ func Test_Routes(t *testing.T) {
 				err = json.Unmarshal(body, &modelList)
 				assert.Nil(t, err)
 
+				assert.NotNil(t, modelList.Models)
 				assert.Equal(t, 0, len(modelList.Models))
 			},
 		},