fix: Guard access to window in helpers.dom.ts for browserless rendering

[unverbraucht]

I'm rendering chart.js on node via chartjs-node-canvas. I found an issue where the window superglobal was accessed even if the DOM wasn't available and have fixed it with a simple guard.

The access is in a try-catch-block which seems to check if event listeners can be added and removed. Since access to window outside of the browser doesn't raise a catcheable error this would fail fatally outside of the browser.

I can provide a test project with chartjs-node-canvas but for this simple fix I was hoping that the bugfix is acceptable without it :)