RFR: 8372949: ClassWriter.writeClassAttributes() can throw NPE

[Daisuke Yamazaki]

Hello maintainers,

This change prevents a possible NPE in [`ClassWriter.writeClassWttributes()`](https://github.com/openjdk/jdk/blob/5f083abafc7abfaa46ddd053668cdfbfd2ad8a87/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/tools/jcore/ClassWriter.java#L891).
[`ConstantPool.getBootstrapMethodAt()`](https://github.com/openjdk/jdk/blob/5f083abafc7abfaa46ddd053668cdfbfd2ad8a87/src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/oops/ConstantPool.java#L479) now returns an empty array instead of `null` when no bootstrap methods are available.

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Commit messages:
 - 8372949: Prevent NPE in getBootstrapMethodAt by returning an empty array for null offsets or bootstrap methods

Changes: https://git.openjdk.org/jdk/pull/28690/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28690&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372949
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28690.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28690/head:pull/28690

PR: https://git.openjdk.org/jdk/pull/28690