RFR: 8366522: CodeSource.getCodeSigners() throws NPE within empty certs

[Kirill Shirokov]

The new tests cover only NPE scenarios.

-------------

Commit messages:
 - 8366522: CodeSource.getCodeSigners() throws NPE within empty certs

Changes: https://git.openjdk.org/jdk/pull/27105/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366522
  Stats: 81 lines in 2 files changed: 80 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27105/head:pull/27105

PR: https://git.openjdk.org/jdk/pull/27105