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/assets/TEAPOT

@@ -0,0 +1,6 @@
+This "Utah teapot" photograph is licensed under the Creative Commons
+Attribution-Share Alike 3.0 Unported license:
+  https://creativecommons.org/licenses/by-sa/3.0/deed.en
+
+Original source of work:
+  https://commons.wikimedia.org/wiki/File:Utah_teapot_simple_2.png

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