Make stdout and stderr properties undefined instead of null

[ehmicky]

The stdout and stderr properties can be undefined with execa(), e.g. when the option stdout: 'ignore' was used.

However with execa.sync(), it is null instead. This change increases consistency by setting those to undefined instead.

[ehmicky]

Fixed.

[sindresorhus]

Tests are failing on Windows.

[ehmicky]

The Windows tests are fixed on CI now.

The issue is that exec.sync() should be an empty string when the child process emits an error event (as opposed to throwing an exception right away). The only way I manage to trigger that event is by firing a command that does not exist. I even looked up the code in core Node.js and libuv to get ideas on how to trigger this, but I get no ideas. And firing a command that does not exist produces some output on Windows, but not on Unix. But I fixed the test accordingly.