Customize: Allow (optional) `url` parameter to be omitted in intercepted calls...

Customize: Allow (optional) `url` parameter to be omitted in intercepted calls to `history.pushState()` and `history.replaceState()` in customize preview.

Fixes issue where calls without the `url` parameter erroneously end up rewriting the location path to `/undefined`.

Props Christian1012, westonruter.
Fixes #39175.

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git-svn-id: https://core.svn.wordpress.org/trunk@39487 1a063a9b-81f0-0310-95a4-ce76da25c4cd