RFR: 8353845: com.sun.javafx.css.BitSet.equals(null) throws NPE

[Michael Strauß]

Fixes the bug that `BitSet.equals(null)` throws NPE.

A single reviewer should be sufficient.

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Commit messages:
 - fix equals(null)
 - failing test

Changes: https://git.openjdk.org/jfx/pull/1766/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1766&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353845
  Stats: 16 lines in 3 files changed: 9 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1766.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1766/head:pull/1766

PR: https://git.openjdk.org/jfx/pull/1766