uploads: make upload_media_assets.media_asset_id nullable.

Make upload_media_assets.media_asset_id nullable in order to support
multi-file uploads. The media asset will be null while the image is
still being downloaded from the source.

db/migrate/20220210200157_set_media_asset_to_nullable_on_upload_media_assets.rb

@@ -0,0 +1,5 @@
+class SetMediaAssetToNullableOnUploadMediaAssets < ActiveRecord::Migration[7.0]
+  def change
+    change_column_null :upload_media_assets, :media_asset_id, true
+  end
+end