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.go

@@ -725,7 +725,7 @@ func (s *Server) ListModelsHandler(c *gin.Context) {
 		return
 	}
 
-	var models []api.ModelResponse
+	models := []api.ModelResponse{}
 	if err := filepath.Walk(manifests, func(path string, info os.FileInfo, _ error) error {
 		if !info.IsDir() {
 			rel, err := filepath.Rel(manifests, path)