Fix Error when creating empty RSS feed on PHP 8

When the feed is empty, ObjectIterator::size() throws a TypeError
exception on PHP 8.0+ (and a Warning on PHP 7.2+) due to calling count()
on $this->objectlist->objects, which is null in this case.

Fixes 0033634

Signed-off-by: Damien Regad dregad@mantisbt.org

• Rewrote original commit message
• Fixed whitespace
• Updated library\README.md
• Removed closing ?> tag