RFR: 8285696: AlgorithmConstraints:permits not throwing IllegalArgumentException when 'alg' is null

[Daniel Jeliński]

Please review this follow up to #8349.

As JCK pointed out, `permits` is supposed to throw IAE on null input. However, now that we're looking up the result in a `ConcurrentHashMap`, a `NullPointerException` is thrown. This patch restores the original behavior.

Verified that the JCK test passes with this change.

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Commit messages:
 - Permits should throw IAE on null input

Changes: https://git.openjdk.java.net/jdk/pull/8427/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8427&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285696
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8427.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8427/head:pull/8427

PR: https://git.openjdk.java.net/jdk/pull/8427