RFR: 8375070: NPE in Scene.ClickGenerator::preProcess when mouse button is none

[Christopher Schnick]

A straightforward fix to just include MouseButton.NONE in the click counter map in cases weird mouse button events are sent

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Commit messages:
 - Remove excessive comment
 - Fix possible NPE

Changes: https://git.openjdk.org/jfx/pull/2034/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2034&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375070
  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2034.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2034/head:pull/2034

PR: https://git.openjdk.org/jfx/pull/2034