RFR: 8310232: java.time.Clock$TickClock.millis() fails in runtime when tick is 1 microsecond

[Naoto Sato]

Fixing the `/ by zero` exception with tick durations less than a millisecond.

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Commit messages:
 - refactor return statement
 - 8310232: java.time.Clock$TickClock.millis() fails in runtime when tick is 1 microsecond

Changes: https://git.openjdk.org/jdk/pull/14657/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14657&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310232
  Stats: 13 lines in 2 files changed: 10 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/14657.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14657/head:pull/14657

PR: https://git.openjdk.org/jdk/pull/14657