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

[Andrey Turbanov]

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

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Commit messages:
 - [PATCH] Avoid redundant Hashtable.containsKey call in CodeSource.readObject

Changes: https://git.openjdk.org/jdk/pull/21753/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21753&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343194
  Stats: 6 lines in 1 file changed: 0 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21753.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21753/head:pull/21753

PR: https://git.openjdk.org/jdk/pull/21753