fix(overlay-container): overlay container throws an error if its wasn't set initially

[djleonskennedy]

fix(overlay-container): if initial theme overlay container was not set after toggle the class error occurred

We're using ngrx and we have themes in store, our initial state is empty (''), when we try switch (e.g to dark and back to empty error occurs Uncaught DOMException: Failed to execute 'contains' on 'DOMTokenList': The token provided must not be empty. this PR able to fix it

test(overlay): test for safe toggle theme of theme wasn't initialized

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[djleonskennedy]

I signed it!

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[djleonskennedy]

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[jelbourn]

Fixed in #6306

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