RFR: 8292317: Missing null check for Iterator.forEachRemaining implementations

[Jaikiran Pai]

Can I please get a review of this change which proposes to fix https://bugs.openjdk.org/browse/JDK-8292317?

The `java.util.Iterator` has a `forEachRemaining(Consumer<? super E> action)` method. As per its contract, the implementations are expected to throw a `NullPointerException` if the passed `action` is `null`.

`java.util.Collections` has a couple of places where it wraps the passed `action` into another and passes that wrapped `action` to the underlying `java.util.Iterator`'s default `forEachRemaining` implementation which is:


Objects.requireNonNull(action);
  while (hasNext())
      action.accept(next());

Since the passed `action` is now a non-null wrapper, the implementation goes ahead and advances the iterator to the next entry and invokes on the non-null `action` to carry out its action. That non-null wrapper action then calls the `null` user passed action and runs into an expected `NullPointerException`. However, at this point the iterator and already advanced and that is what the bug is.

The commit in this PR introduces a trivial null check on the `action` very early in the call even before wrapping such an `action`. This prevents any further logic to execute if `action` is null.

New test methods have been added to the existing test class `test/jdk/java/util/Collections/DelegatingIteratorForEachRemaining.java`. This test class was introduced in https://bugs.openjdk.org/browse/JDK-8205184 where this delegation logic was added for the `forEachRemaining` methods. These new test methods reproduce the failure and verify the fix.

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Commit messages:
 - 8292317: Missing null check for Iterator.forEachRemaining implementations

Changes: https://git.openjdk.org/jdk/pull/11154/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11154&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292317
  Stats: 41 lines in 2 files changed: 40 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11154.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11154/head:pull/11154

PR: https://git.openjdk.org/jdk/pull/11154