fix: manual_unwrap_or_default suggests error when expression is a None variant

[t-webber]

Suggesting error when expression is none : None.unwrap_or_default() raises an error, as explained in the following issue:

fixes #12670

changelog: [manual_unwrap_or_default]: the lint is ignored when matching None, as the unwrap_or_default method can't infer the type of a None variant.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Jarcho (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git rebase -i master
$ # delete any merge commits in the editor that appears
$ git push --force-with-lease

The following commits are merge commits:

• f97d38e

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git rebase -i master
$ # delete any merge commits in the editor that appears
$ git push --force-with-lease

The following commits are merge commits:

• 1d9e0cd
• f97d38e

[J-ZhengLi]

@y21 since you have already reviewed this, do you want to take it over? 😄

(Jarcho has SO many open PR assigned already, and judging by their contribution graph, I assume they might've been busy lately?)