RFR: 7820: Add validations for master password

[Suchita Chaturvedi]

This PR adds password validations for master password field.

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Commit messages:
 - 7820: Add validations for master password

Changes: https://git.openjdk.org/jmc/pull/406/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=406&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7820
  Stats: 21 lines in 3 files changed: 11 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jmc/pull/406.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/406/head:pull/406

PR: https://git.openjdk.org/jmc/pull/406