This implements an optional $skipWhenEmpty parameter (defaulting to false) for the #[DataProvider] and #[DataProviderExternal] attributes. When set to true, a data provider that returns an empty iterable will cause the test to be skipped instead of triggering an error.

Usage:

#[DataProvider('provider', skipWhenEmpty: true)]
public function testSomething(string $value): void
{
    // ...
}

Note: Parameter names are not covered by the backward compatibility promise for PHPUnit.

When all data providers for a test method return no data and at least one of them has skipWhenEmpty: true, the test is created as a single test case that skips itself with the message "The data provider for this test provided no data, which is explicitly permitted". The default behavior (error on empty data provider) is unchanged.

Closes #6404.