feat(diagnostics): use 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

@@ -76,7 +76,7 @@ pub mod ext {
         expr_from_text(r#""""#)
     }
     pub fn empty_char() -> ast::Expr {
-        expr_from_text("''")
+        expr_from_text("'\x00'")
     }
     pub fn default_bool() -> ast::Expr {
         expr_from_text("false")