RFR: 8296878: Document Filter attached to JPasswordField and setText("") is not cleared instead inserted characters replaced with unicode null characters

[Sergey Bylokhov]

The usage of DocumentFilter in the JPasswordField is excluded from the [JDK-8258373](https://bugs.openjdk.org/browse/JDK-8258373). That will be responsibility of the application to cleanup the password if DocumentFilter is set.

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Commit messages:
 - Update OldPasswordInDocumentFilter.java
 - 8296878: Document Filter attached to JPasswordField and setText("") is not cleared instead inserted characters replaced with unicode null characters

Changes: https://git.openjdk.org/jdk/pull/11281/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11281&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296878
  Stats: 94 lines in 2 files changed: 79 ins; 2 del; 13 mod
  Patch: https://git.openjdk.org/jdk/pull/11281.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11281/head:pull/11281

PR: https://git.openjdk.org/jdk/pull/11281