Fix 'using null as an array offset is deprecated' on PHP 8.5

[imorland]

Line::concat() tests the array before it tests the index:

$fi = $source->fileIndex;
$ni = $source->nameIndex;
if ((isset($mSources[$fi])) && ($fi !== null)) {
    $source->fileIndex = $mSources[$fi];
}
if ((isset($mNames[$ni])) && ($ni !== null)) {
    $source->nameIndex = $mNames[$ni];
}

isset($mSources[$fi]) evaluates the offset first, so with $fi null the $fi !== null check is never reached — it can't do anything. $fi and $ni are null whenever a position has no source file or no name, which is routine, and PHP 8.5 reports every occurrence:

PHP Deprecated:  Using null as an array offset is deprecated, use an empty string instead in src/parsing/Line.php on line 495

Swapping the operands can't change behaviour, since isset() on a null offset is already false.

I found this in Flarum, where we concatenate a sourcemap per asset when building the frontend bundle. On a debug install with display_errors on it's hundreds of lines per request — flarum/framework#4894 has the detail.

Checks

• Test suite passes on 8.1, 8.2, 8.3, 8.4 and 8.5 — 147 tests, 387 assertions.
• concat() output is byte-identical before and after, same input.
• No deprecations left in src on 8.5. The 7 that remain are all in axy/errors and axy/codecs-base64vlq, which I've mentioned in the issue.
• phpcs clean.

Closes #12