RFR: 8377393: RichTextArea: failed to export null color attribute value

[Andy Goryachev]

RichEditorDemo exposed a logical flaw in the RichTextFormatHandler responsible for copy/save/export code. Technically speaking, this is a product bug in the RichTextArea, though it can only be reproduced with RichEditorDemo.

To reproduce:

- open RichEditorDemo app
- type 1, tab, 2, tab, 3
- select the first line
- copy (command-C or ctrl-C)
- paste (command-V or ctrl-V)

--> nothing gets pasted because the RichEditorDemo toolbar sets the text color to null, which breaks the attribute

Also, attempt to save this file produces corrupted output.

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Commit messages:
 - export null color value

Changes: https://git.openjdk.org/jfx/pull/2067/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2067&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377393
  Stats: 13 lines in 2 files changed: 6 ins; 1 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/2067.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2067/head:pull/2067

PR: https://git.openjdk.org/jfx/pull/2067