core(image-usage): add null check for parentElement

[AkshayIyer12]

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[AkshayIyer12]

I signed it!

[googlebot]

CLAs look good, thanks!

[wardpeet]

@AkshayIyer12 Thanks for fixing this bug! Do you have a webpage where this audit fails? is it possible to create a unit test for this one?

[patrickhulce]

@wardpeet yeah the details are in #3731 :)

unfortunately a unit test is tricky since its eval'd in the browser, but a smoke test would be nice

[wardpeet]

LGTM

[patrickhulce]

thanks for taking this on @AkshayIyer12 !! 🎉