twitter card: handle case where image has no alt text

lib/pleroma/web/metadata/providers/twitter_card.ex

@@ -79,8 +79,7 @@ defp build_attachments(id, %{data: %{"attachment" => attachments}}) do
                    name: "twitter:image",
                    content: MediaProxy.url(url["href"])
                  ], []},
-                {:meta, [property: "twitter:image:alt", content: truncate(attachment["name"])],
-                 []}
+                {:meta, [name: "twitter:image:alt", content: truncate(attachment["name"])], []}
                 | acc
               ]
               |> maybe_add_dimensions(url)
@@ -133,6 +132,8 @@ defp maybe_add_dimensions(metadata, url) do
     end
   end
 
+  defp truncate(nil), do: ""
+
   defp truncate(text) do
     # truncate to 420 characters
     # see https://developer.twitter.com/en/docs/twitter-for-websites/cards/overview/markup