feat(diagnostics): use Default::default() expression instead of todo! when missing fields

Signed-off-by: Benjamin Coenen <5719034+bnjjj@users.noreply.github.com>

crates/syntax/src/ast/make.rs

@@ -59,6 +59,9 @@ pub mod ext {
     pub fn expr_todo() -> ast::Expr {
         expr_from_text("todo!()")
     }
+    pub fn expr_default() -> ast::Expr {
+        expr_from_text("Default::default()")
+    }
     pub fn empty_block_expr() -> ast::BlockExpr {
         block_expr(None, None)
     }