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

app/models/form/admin_settings.rb

@@ -67,7 +67,7 @@ class Form::AdminSettings
     KEYS.each do |key|
       value = instance_variable_get("@#{key}")
 
-      if UPLOAD_KEYS.include?(key)
+      if UPLOAD_KEYS.include?(key) && !value.nil?
         upload = SiteUpload.where(var: key).first_or_initialize(var: key)
         upload.update(file: value)
       else

app/models/site_upload.rb

@@ -18,6 +18,7 @@ class SiteUpload < ApplicationRecord
   has_attached_file :file
 
   validates_attachment_content_type :file, content_type: /\Aimage\/.*\z/
+  validates :file, presence: true
   validates :var, presence: true, uniqueness: true
 
   before_save :set_meta