RFR: 8370253: CodeArea: NPE on copy

[Andy Goryachev]

Fixes a missing `null` check in `CodeArea.copy()` when the `syntaxHighlighter` property is set. 

Also
- made sure other callers check for null
- removed dead code

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Commit messages:
 - copy

Changes: https://git.openjdk.org/jfx/pull/1942/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1942&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370253
  Stats: 91 lines in 3 files changed: 70 ins; 18 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1942.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1942/head:pull/1942

PR: https://git.openjdk.org/jfx/pull/1942