Improvements to image upload validation and creation (#10431)

* Check if image value is nil? before creating an image

Check if uploaded images aren't nil before creating SiteUpload models
for them.

* Validate presence of file in SiteUpload

* Fix file presence validation

* Fabricate SiteUpload#file

* Add link to Creative Commons license

spec/fabricators/site_upload_fabricator.rb

@@ -1,2 +1,3 @@
 Fabricator(:site_upload) do
+  file { File.open(File.join(Rails.root, 'spec', 'fabricators', 'assets', 'utah_teapot.png')) }
 end