RFR: 8258422: Cleanup unnecessary null comparison before instanceof check in java.base

[Aleksei Efimov]

On Mon, 11 Jan 2021 17:12:24 GMT, Andrey Turbanov <github.com+741251+turbanoff at openjdk.org> wrote:

>> Hi @turbanoff,
>> This PR is ready for integration - `JDK-8258657` has been resolved.
>> You can proceed with issuing `integrate` bot command. Then I will `sponsor` it.
>> But before that, I would like to ask if you would like to take on board the changes to `HttpClientImpl`? Alternatively, I can follow it up separately and reapply the backed-out change under different bugID.
>
> @AlekseiEfimov `HttpClientImpl` is not in `java.base` module. So I think it's better to not touch it under this PR.

To double check that docs build will be stable after integration the following actions have been performed:
- The pull request branch was locally synced-up with the latest changes from `master` (`JDK-8258657` is part of them)
- Verified that local  `docs-reference-api-javadoc` build completes successfully

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PR: https://git.openjdk.java.net/jdk/pull/20