RFR: 8299522: Incorrect size of Approve button in custom JFileChooser [v6]

[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.

Tejesh R has updated the pull request incrementally with one additional commit since the last revision:

  Updated based on review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11901/files
  - new: https://git.openjdk.org/jdk/pull/11901/files/ede54bad..14b358c1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11901&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11901&range=04-05

  Stats: 55 lines in 2 files changed: 23 ins; 15 del; 17 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