Fix 500 error when try to create project snippet without content

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/snippets_controller.rb

@@ -30,9 +30,14 @@ class Projects::SnippetsController < Projects::ApplicationController
   def create
     @snippet = CreateSnippetService.new(@project, current_user,
                                         snippet_params).execute
+
+    if @snippet.valid?
       respond_with(@snippet,
                    location: namespace_project_snippet_path(@project.namespace,
                                                             @project, @snippet))
+    else
+      render :new
+    end
   end
 
   def edit