Core: Fix NPE in CloseableIterable.close()

[nastra]

This can be reproduced by running:

CloseableIterable<Object> closeableIterable = CloseableIterable.concat(Collections.emptyList());
closeableIterable.iterator();
// throws a NPE here
closeableIterable.close();

[findepi]

@nastra thank you for the fix.

Note that build isn't green

[nastra]

@nastra thank you for the fix.

Note that build isn't green

This is because CI is currently broken and will be fixed by #6314

[stevenzwu]

nit: this method name can be more descriptive, sth like testCloseWithEmptyIterables

[nastra]

I specifically avoid naming it like this, because the important piece where this NPE is happening is the usage of .iterator() before doing a .close()