bootstrap: Don't panic if codegen-backends is set to empty

Author: GuillaumeGomez

src/bootstrap/src/core/build_steps/test.rs

@@ -1849,8 +1849,13 @@ HELP: You can add it into `bootstrap.toml` in `rust.codegen-backends = [{name:?}
         } else {
             // Tells compiletest which codegen backend to use.
             // It is used to e.g. ignore tests that don't support that codegen backend.
-            cmd.arg("--default-codegen-backend")
-                .arg(builder.config.default_codegen_backend(compiler.host).unwrap().name());
+            cmd.arg("--default-codegen-backend").arg(
+                builder
+                    .config
+                    .default_codegen_backend(compiler.host)
+                    .unwrap_or(&CodegenBackendKind::Llvm)
+                    .name(),
+            );
         }
 
         if builder.build.config.llvm_enzyme {