[jdk11u-dev] RFR: 8319436: Proxy.newProxyInstance throws NPE if loader is null and interface not visible from class loader

[Ben Taylor]

Clean backport of [JDK-8319436](https://bugs.openjdk.org/browse/JDK-8319436) to resolve a regression introduced in 11.0.20. Included tests pass locally on linux x64.

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Commit messages:
 - Backport 8eb6f617b3f8e2a990e4244af4e56287f9741b20

Changes: https://git.openjdk.org/jdk11u-dev/pull/2454/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=2454&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319436
  Stats: 143 lines in 2 files changed: 20 ins; 77 del; 46 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/2454.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev.git pull/2454/head:pull/2454

PR: https://git.openjdk.org/jdk11u-dev/pull/2454