RFR: 8349383: (fs) FileTreeWalker.next() superfluous null check of visit() return value

[Brian Burkhalter]

Remove the `do-while` loop which was conditional on `Event` being `null` when it can never be so.

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Commit messages:
 - 8349383: (fs) FileTreeWalker.next() superfluous null check of visit() return value

Changes: https://git.openjdk.org/jdk/pull/23453/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349383
  Stats: 41 lines in 1 file changed: 6 ins; 12 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/23453.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23453/head:pull/23453

PR: https://git.openjdk.org/jdk/pull/23453