PHPUnit fails to infer zero count for an EmptyIterator instance.

[sanmai]

Q	A
PHPUnit version	8.2.3
PHP version	7.3.4
Installation Method	Composer

This assertion should not fail:

$this->assertCount(0, new \EmptyIterator());

But it fails with an error:

1) Example::testEmptyIterator
BadMethodCallException: Accessing the key of an EmptyIterator

On the other hand this assertion works without a hitch:

$this->assertSame(0, iterator_count(new \EmptyIterator()));

These seems to be the problematic lines:

phpunit/src/Framework/Constraint/Count.php

Lines 71 to 84 in 6efdf39

	$key = $iterator->key();
	$count = \iterator_count($iterator);
	// Manually rewind $iterator to previous key, since iterator_count
	// moves pointer.
	if ($key !== null) {
	$iterator->rewind();
	while ($iterator->valid() && $key !== $iterator->key()) {
	$iterator->next();
	}
	}
	return $count;

A Travis CI build with this failure.