fixes issue serializing a web_page_template with no main widgets
it would return undefined method map for nil. this will return an empty array instead of nil
Merge pull request #356 from G5/main-widgets-issue

fixes issue serializing a web_page_template with no main widgets
Merge branch 'master' of github.com:g5search/g5-content-management-system into comarketing-page-69118558