Add NotNullWhen(true) on Directory/File.Exists

[huoyaoyuan]

Closes #47573 .

Note: there were already annotations on ref of Directory and impl on File. Doesn't ApiCompact check this?

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, to please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[stephentoub]

there were already annotations on ref of Directory and impl on File. Doesn't ApiCompact check this?

@ericstj, @safern?

[safern]

APICompat doesn't have a way to check nullable annotations yet, however we do plan on fixing this for the 6.0.0 release. Attribute differences should be flagged by APICompat just fine.