Rollup merge of #139848 - nnethercote:kw-Empty-5, r=compiler-errors

Reduce kw::Empty usage, part 5

Another step towards https://github.com/rust-lang/rust/issues/137978.

r? `@davidtwco`

compiler/rustc_symbol_mangling/src/v0.rs

@@ -776,7 +776,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
                                     self.push_disambiguator(
                                         disambiguated_field.disambiguator as u64,
                                     );
-                                    self.push_ident(field_name.unwrap_or(kw::Empty).as_str());
+                                    self.push_ident(field_name.unwrap().as_str());
 
                                     field.print(self)?;
                                 }