Fix an illegal array access in DC_Table when expanding the tree (see #…

…2805)

Description
-----------

| Q                | A
| -----------------| ---
| Fixed issues     | Fixes #2797 
| Docs PR or issue | -

I split the PR in two commits:
* one that *only* formats the code so that there is any chance of reviewing this:
a1a16fe
* one that applies the fixes:
9bcedb7

(If you want to, I can collapse the whole thing into a oneline-mega-expression again in a third one…)

Commits
-------

a1a16fe format call
9bcedb7 fix illegal array access
2f22b2f collapse expression

core-bundle/src/Resources/contao/drivers/DC_Table.php

@@ -3844,7 +3844,7 @@ public function ajaxTreeView($id, $level)
 
 		for ($i=0, $c=\count($arrIds); $i<$c; $i++)
 		{
-			$return .= ' ' . trim($this->generateTree($table, $arrIds[$i], array('p'=>$arrIds[($i-1)], 'n'=>$arrIds[($i+1)]), $hasSorting, $margin, ($blnClipboard ? $arrClipboard : false), ($id == $arrClipboard['id'] || (\is_array($arrClipboard['id']) && \in_array($id, $arrClipboard['id'])) || (!$blnPtable && !\is_array($arrClipboard['id']) && \in_array($id, $this->Database->getChildRecords($arrClipboard['id'], $table)))), $blnProtected));
+			$return .= ' ' . trim($this->generateTree($table, $arrIds[$i], array('p'=>($arrIds[$i-1] ?? null), 'n'=>($arrIds[$i+1] ?? null)), $hasSorting, $margin, ($blnClipboard ? $arrClipboard : false), $arrClipboard !== null && ($id == $arrClipboard['id'] || (\is_array($arrClipboard['id']) && \in_array($id, $arrClipboard['id'])) || (!$blnPtable && !\is_array($arrClipboard['id']) && \in_array($id, $this->Database->getChildRecords($arrClipboard['id'], $table)))), $blnProtected));
 		}
 
 		return $return;