RFR: 8264514: HexFormat implementation tweaks

[Raffaello Giulietti]

(Changed to new branch in personal fork)

Please review these small tweaks.
For background information see [1](https://mail.openjdk.java.net/pipermail/core-libs-dev/2021-March/075822.html)

Greetings
Raffaello

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Commit messages:
 - 8264514: HexFormat implementation tweaks

Changes: https://git.openjdk.java.net/jdk/pull/3381/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3381&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264514
  Stats: 67 lines in 1 file changed: 10 ins; 26 del; 31 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3381.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3381/head:pull/3381

PR: https://git.openjdk.java.net/jdk/pull/3381