Flux#next throws NoSuchElementException on a non-finite empty iterator

[jzheaux]

Spring Security's R2dbcReactiveOAuth2AuthorizedClientServiceTests started failing on the 3.5.1-SNAPSHOT recently.

It appears to coincide with the changes in this commit (67cc87a).

Here is a quick test that fails that is a minimal representation of what the Spring Security tests are doing:

@Test
public void testFluxIterable() {
    Iterable<String> iterable = mock(Iterable.class);
    given(iterable.spliterator()).willReturn(mock(Spliterator.class));
    Flux.fromIterable(iterable).next().block();
}

The Javadoc for Flux#next says

Emit only the first item emitted by this Flux, into a new Mono. If called on an empty Flux, emits an empty Mono.

I would expect the above test to pass, but it instead throws a NoSuchElementException.