RFR: 8287053: Avoid redundant HashMap.containsKey calls in ZoneInfoFile.getZoneInfo0

[Andrey Turbanov]

Instead of pair `HashMap.containsKey`/`HashMap.get` method calls, we can use single call `HashMap.getOrDefault`.
It's faster and clearer.

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Commit messages:
 - [PATCH] Avoid redundant HashMap.containsKey calls in ZoneInfoFile

Changes: https://git.openjdk.java.net/jdk/pull/8487/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8487&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287053
  Stats: 6 lines in 1 file changed: 0 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8487.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8487/head:pull/8487

PR: https://git.openjdk.java.net/jdk/pull/8487