getApproved require the given token ID exist

[lamengao]

🚀 Description

In the ERC-721 standard the getApproved comment mentioned "Throws if _tokenId is not a valid NFT".

    /// @notice Get the approved address for a single NFT
    /// @dev Throws if `_tokenId` is not a valid NFT.
    /// @param _tokenId The NFT to find the approved address for
    /// @return The approved address for this NFT, or the zero address if there is none
    function getApproved(uint256 _tokenId) external view returns (address);

This PR make getApproved conforms to ERC721 standard in better terms.

• 📘 I've reviewed the OpenZeppelin Contributor Guidelines
• ✅ 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).

[nventuro]

Great catch @lamengao, thanks!

[nventuro]

I find the current phrasing a bit confusing: maybe we could merge the context and it blocks, and change the description to something like 'causes getApproved to revert'?

We also may want to add a test block for getApproved, explicitly checking for this, though that could be considered part of #1148.

[come-maiz]

@nventuro I made a quick attempt at clarifying the descriptions. And added a comment on #1148 to not forget about the getApproved tests.