RFR: 8303478: DatePicker throws uncatchable exception on tab out from garbled text

[brunesto]

The fix prevents the DatePicker from losing focus if the date is not parsable.

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Commit messages:
 - DatePicker restores the previous value when focus is lost and the date is not parseable
 - method visibility
 - whitespaces
 - 8303478: DatePicker throws uncatchable exception on tab out from garbled text

Changes: https://git.openjdk.org/jfx/pull/1274/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1274&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303478
  Stats: 228 lines in 2 files changed: 227 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1274.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1274/head:pull/1274

PR: https://git.openjdk.org/jfx/pull/1274