fix empty import URL errors

app/models/project.rb

@@ -494,7 +494,7 @@ class Project < ActiveRecord::Base
   end
 
   def import_url
-    if import_data && super
+    if import_data && super.present?
       import_url = Gitlab::UrlSanitizer.new(super, credentials: import_data.credentials)
       import_url.full_url
     else