[jdk11u-dev] RFR: 8216408: XMLStreamWriter setDefaultNamespace(null) throws NullPointerException

[Amos Shi]

Backport of [JDK-8216408](https://bugs.openjdk.org/browse/JDK-8216408)

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Commit messages:
 - Backport 1ebe11a28da71848f2066c53707a7ce9ece9395c

Changes: https://git.openjdk.org/jdk11u-dev/pull/2356/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2356&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8216408
  Stats: 29 lines in 2 files changed: 17 ins; 5 del; 7 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2356.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2356/head:pull/2356

PR: https://git.openjdk.org/jdk11u-dev/pull/2356