test: handle blank shells in test-os.js

The shell in /etc/passwd can be blank, in which case the user is given
the default shell. Handle this by only checking the shell contains a
path separator if the string isn't empty.

PR-URL: nodejs/node#16287
Fixes: nodejs/node#15684
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

test/parallel/test-os.js

@@ -208,7 +208,11 @@ if (common.isWindows) {
 } else {
   is.number(pwd.uid);
   is.number(pwd.gid);
-  assert.ok(pwd.shell.includes(path.sep));
+  assert.strictEqual(typeof pwd.shell, 'string');
+  // It's possible for /etc/passwd to leave the user's shell blank.
+  if (pwd.shell.length > 0) {
+    assert(pwd.shell.includes(path.sep));
+  }
   assert.strictEqual(pwd.uid, pwdBuf.uid);
   assert.strictEqual(pwd.gid, pwdBuf.gid);
   assert.strictEqual(pwd.shell, pwdBuf.shell.toString('utf8'));