RFR: 8343194: Avoid redundant Hashtable.containsKey call in CodeSource.readObject

[Sean Mullan]

On Tue, 29 Oct 2024 08:15:58 GMT, Andrey Turbanov <aturbanov at openjdk.org> wrote:

> `Hashtable` doesn't allow `null` values.
> It means we can replace containsKey+get with get+null check.
> It's clearer and a bit faster.

Looks fine.

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Marked as reviewed by mullan (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21753#pullrequestreview-2402830781