RFR: 8299522: JFilechooser open button size is incorrectly shown

[Tejesh R]

FileChooser Open/Approve button size is shown incorrectly when no Approve button text is set in `CUSTOM_DIALOG` type. Reason being that no default Approve Button text is returned during Dialog Type Property change. Since `null` is returned as Button string the Button size is incorrectly shown. The fix here addresses the issue by adding a default Approve Button Text when no text is set explicitly in case of `CUSTOM_DIALOG` type.
Automated test is attached which has been tested with multiple test runs.

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Commit messages:
 - fix + Automated Test

Changes: https://git.openjdk.org/jdk/pull/11901/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11901&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299522
  Stats: 150 lines in 3 files changed: 150 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11901.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11901/head:pull/11901

PR: https://git.openjdk.org/jdk/pull/11901