RFR: 8300549: JFileChooser Approve button tooltip is null in Aqua L&F in CUSTOM_DIALOG mode

[Tejesh R]

_JFilechooser_ returns _null_ for Approve button tool tip in `CUSTOM_DIALOG` mode (`getToolTipText()`). The fallback text has been set to "Choose" text. Observation found in validating the test of PR - #11901.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/12198/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12198&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300549
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12198.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12198/head:pull/12198

PR: https://git.openjdk.org/jdk/pull/12198