RFR: 8318150: StaticProxySelector.select should not throw NullPointerExceptions

[Daniel Jeliński]

`ProxySelector.select` and `connectFailed` methods should throw `IllegalArgumentException`s on null parameters.

Additionally, the `select` method doesn't need to be synchronized - the selector is immutable.

Tier2 green.

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Commit messages:
 - Fix connectFailed, add test
 - Improve null handling

Changes: https://git.openjdk.org/jdk/pull/16199/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16199&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318150
  Stats: 74 lines in 2 files changed: 19 ins; 24 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/16199.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16199/head:pull/16199

PR: https://git.openjdk.org/jdk/pull/16199