Don't show url if No last deployed commit exist

[Amr-Aboshama]

Fix https://github.com/shop/issues/issues/1614

Problem:
When user open stack page on shipit, in some state while deploying View tries to get next commit to deploy and add a URL with its SHA but it return a nil commit, so it doesn't have SHA property that raise an NoMethodError: undefined method sha' for nil`.

Fix:

• Get the next commit.
• Check if next commit exists, show add its URL to the View.
• Otherwise, show the view without the URL