RFR: 8347715: RichTextArea Follow-up: Minor Bugs

[Andy Goryachev]

Found during writing the API tests:

- setUseContentHeight/Width (true -> on)
- remove unnecessary non-nullable limitation for caretBlinkProperty

-------------

Commit messages:
 - 8347715: RichTextArea Follow-up: Minor Bugs

Changes: https://git.openjdk.org/jfx/pull/1676/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1676&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347715
  Stats: 24 lines in 2 files changed: 3 ins; 12 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/1676.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1676/head:pull/1676

PR: https://git.openjdk.org/jfx/pull/1676