Filter out legacy assets when checking if all assets uploaded

If the asset doesn't respond to `variant`, it's a legacy asset
(PDF thumbnail) and we want to filter it out.

I did consider applying this filter more generally by overloading
the `assets` call as below, but that breaks a bunch of tests - so
any filtering needs to be applied on a per-case basis.

```
  def assets
    super.select { |asset| asset.variant.present? }
  end
```

app/models/attachment_data.rb

@@ -60,7 +60,7 @@ def indexable?
   end
 
   def all_asset_variants_uploaded?
-    asset_variants = assets.map(&:variant).map(&:to_sym)
+    asset_variants = assets.map(&:variant).compact.map(&:to_sym)
 
     (%i[original] - asset_variants).empty?
   end

test/unit/app/models/attachment_data_test.rb

@@ -432,6 +432,21 @@ class AttachmentDataTest < ActiveSupport::TestCase
     assert attachment_data.all_asset_variants_uploaded?
   end
 
+  test "#all_asset_variants_uploaded? skips over any legacy asset variants present" do
+    attachment_data = build(:attachment_data)
+
+    original_pdf_asset = build(:asset, asset_manager_id: "asset_manager_id_original", variant: Asset.variants[:original], filename: attachment_data.filename)
+    # Unable to set `variant: :thumbnail` here (which is what we really want to test)
+    # as it's not a valid enum and there seems to be no way of stubbing it.
+    # But setting to `nil` does the same thing for the purposes of this test.
+    legacy_thumbnail_asset = build(:asset, asset_manager_id: "asset_manager_id_thumbnail", variant: nil, filename: "thumbnail_#{attachment_data.filename}.png")
+
+    attachment_data.assets << original_pdf_asset
+    attachment_data.assets << legacy_thumbnail_asset
+
+    assert attachment_data.all_asset_variants_uploaded?
+  end
+
   test "#all_asset_variants_uploaded? returns false if there are no assets" do
     attachment_data = build(:attachment_data_with_no_assets)