RFR: 8296653: ComboBox promptText is not displayed when the value is reset [v3]

[Ziad El Midaoui]

> The issue occurred because the skin marks the button cell as “empty” when the value becomes null.
> With the fix clearing the ComboBox value re-shows the prompt text.
> Tested using the code present in the bug.

Ziad El Midaoui has updated the pull request incrementally with one additional commit since the last revision:

  Fixed test

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1989/files
  - new: https://git.openjdk.org/jfx/pull/1989/files/dd62ca2c..5fd23c04

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1989&range=02
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1989&range=01-02

  Stats: 8 lines in 1 file changed: 3 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jfx/pull/1989.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1989/head:pull/1989

PR: https://git.openjdk.org/jfx/pull/1989