RFR: 8296653: ComboBox promptText is not displayed when the value is reset [v2]

[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 two additional commits since the last revision:

 - Remove space
 - Added Test and fixed comment

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1989/files
  - new: https://git.openjdk.org/jfx/pull/1989/files/81b6231e..dd62ca2c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1989&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1989&range=00-01

  Stats: 25 lines in 2 files changed: 24 ins; 1 del; 0 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