add case when to is equal to 0 an have a previos approve

[NimrodHunter]

Fixes #

🚀 Description

In ethereum/EIPs#721 the official draft of ERC721 token, in the approve function, when the _to address is equal to 0 and have a previous state, the function have to clear the state and trigger the approve event.

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• [] ✅ I've added tests where applicable to test my new functionality.
• 📖 I've made sure that my contracts are well-documented.
• 🎨 I've run the JS/Solidity linters and fixed any issues (npm run lint:all:fix).

[spalladino]

Thanks for the catch, @NimrodHunter! I added this check in #803, with a test case here. I'm closing this issue in favour of that PR.