Fix PHP 8.5 null array key in PhutilRemarkupQuotedBlockRule

Summary: Setting null as an array key is deprecated since PHP 8.5 per https://www.php.net/releases/8.5/en.php: "Using null as an array offset or when calling array_key_exists() is now deprecated. Use an empty string instead."

Test Plan: Unknown. Read the code and check where and how $c is used.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D26670

src/infrastructure/markup/blockrule/PhutilRemarkupQuotedBlockRule.php

···
       if (isset($line[0])) {
         $c = $line[0];
       } else {
-        $c = null;
+        $c = '';
       }
 
       if (isset($no_whitespace[$c])) {