Make Buffer::language_at fall back to Buffer::language

For languages with no grammar (plain text), there
will be no layers.

crates/language/src/syntax_map.rs

@@ -92,6 +92,7 @@ struct SyntaxLayer {
     language: Arc<Language>,
 }
 
+#[derive(Debug)]
 pub struct SyntaxLayerInfo<'a> {
     pub depth: usize,
     pub node: Node<'a>,