PCRE: Unmatched capture groups result inconsistency

Description

Currently, unmatched capture groups are added to the result when they are before matched capture group, but are not added when they are after (ie. unmatched capture group is not followed by any other matched capture group).

The docs about "PREG_UNMATCHED_AS_NULL" flag says:

If this flag is passed, unmatched subpatterns are reported as null; otherwise they are reported as an empty string.

So all capture groups, even the unmatched at the end, should be always present.

The following code:

<?php

preg_match('/(?<foo>y)?x(?<bar>y)?/', 'x', $matches);
print_r($matches);

preg_match('/(?<foo>y)?()x(?<bar>y)?/', 'x', $matches); // notice extra "()"
print_r($matches);

Resulted in this output:

Array
(
    [0] => x
)
Array
(
    [0] => x
    [foo] => 
    [1] => 
    [2] => 
)

But I expected this output instead:

Array
(
    [0] => x
    [foo] => 
    [1] => 
    [2] => 
    [bar] => 
    [3] => 
)
Array
(
    [0] => x
    [foo] => 
    [1] => 
    [2] => 
    [bar] => 
    [3] => 
)

online repro: https://3v4l.org/ra13Q

Start with the documentation for PREG_UNMATCHED_AS_NULL and reproduce both preg_match examples from the issue. Compare the documented behavior with the observed capture-group arrays and determine whether the documentation or PHP behavior is the intended correction. Done means the issue's expected handling of trailing unmatched groups is resolved consistently.