ui: fix incorrectly aligned disc headers

Fix an issue where disc headers would be at the top when no subtitle as
available.

I really thought that TextView.text would be null if I set it to null.
Instead it becomes an empty string, breaking the visbility change. Fix
it by just using the disc value.

Resolves #472.

app/src/main/java/org/oxycblt/auxio/detail/list/AlbumDetailListAdapter.kt

@@ -115,7 +115,7 @@ private class DiscHeaderViewHolder(private val binding: ItemDiscHeaderBinding) :
             binding.discNumber.text = binding.context.getString(R.string.fmt_disc_no, disc.number)
             binding.discName.apply {
                 text = disc.name
-                isGone = text == null
+                isGone = disc.name == null
             }
         } else {
             logD("Disc is null, defaulting to no disc")