Replace dummy initialization with optional chain

lib/worker/base.js

@@ -19,7 +19,7 @@ import {flags, refs, sharedWorkerTeardowns} from './state.cjs';
 import {isRunningInThread, isRunningInChildProcess} from './utils.cjs';
 
 const currentlyUnhandled = setUpCurrentlyUnhandled();
-let runner = Object.create(null);
+let runner;
 
 // Override process.exit with an undetectable replacement
 // to report when it is called from a test (which it should never be).
@@ -37,7 +37,7 @@ async function exit(code, forceSync = false) {
 
 process.exit = new Proxy(realExit, {
 	apply(fn, receiver, args) {
-		channel.send({type: 'internal-error', err: serializeError('Forced exit error', false, new Error('Unexpected process.exit()'), runner.file)});
+		channel.send({type: 'internal-error', err: serializeError('Forced exit error', false, new Error('Unexpected process.exit()'), runner?.file)});
 
 		// Make sure to extract the code only from `args` rather than e.g. `Array.prototype`.
 		// This level of paranoia is usually unwarranted, but we're dealing with test code