test: handle null stdio streams in spawnSyncAndAssert helper

When a test  uses 'pipe' for the stdio but the child process crashes,
the stream will be null. In this case, don't try to stringify it and
instead log an empty string.

Author: joyeecheung

test/common/child_process.js

@@ -80,9 +80,9 @@ function expectSyncExit(caller, spawnArgs, {
   function logAndThrow() {
     const tag = `[process ${child.pid}]:`;
     console.error(`${tag} --- stderr ---`);
-    console.error(stderrStr === undefined ? child.stderr.toString() : stderrStr);
+    console.error(stderrStr === undefined ? (child.stderr?.toString() ?? '') : stderrStr);
     console.error(`${tag} --- stdout ---`);
-    console.error(stdoutStr === undefined ? child.stdout.toString() : stdoutStr);
+    console.error(stdoutStr === undefined ? (child.stdout?.toString() ?? '') : stdoutStr);
     console.error(`${tag} status = ${child.status}, signal = ${child.signal}`);
 
     const error = new Error(`${failures.join('\n')}`);