Controls: Fix undefined args handling in the URL

[evont]

Issue: #17063

What I did

Fix a bug where args were erroneously getting set to undefined when they weren't present in the URL and the page was reloaded.

How to test

cd examples/official-storybook
yarn storybook

1. Open the new story in this PR
2. Change one of the args via controls, observe that the new value gets added to the URL
3. Reload the browser, observe that the new value from step 2 gets restored to that arg.
4. Also observe that the other args remain unchanged. If you perform these steps in next, you'll see that the other args get erroneously set to undefined in both the URL and the args.

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[shilman]

Thank you for the fix @evont @tmeasday. Merging!

[zecka]

Any chance to add this fix in a revision of 6.4 instead of 6.5 ?