do not throw an error when deleting a non-existant prop from draft

[skratchdot]

This should fix the issue found here:
#1015 (comment)

https://codesandbox.io/s/flamboyant-glitter-n2nevz?file=/src/App.js

cc @einarq / @mweststrate

The only concern I have: I did not look into how this library is being transpiled, or if "optional chaining" is used elsewhere in the code. This change did appear to "fix" the new unit test I added

[coveralls]

Pull Request Test Coverage Report for Build 4628822574

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• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 99.697%

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Totals
Change from base Build 4623170994:	-0.1%
Covered Lines:	641
Relevant Lines:	642

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💛 - Coveralls