PHP 8.5: Search: Avoid a null array key in AphrontStackTraceView
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."
```
ERROR 8192: Using null as an array offset is deprecated, use an empty string instead at [/var/www/html/phorge/phorge/src/view/widget/AphrontStackTraceView.php:51]
```
Closes T16372
Test Plan:
Trigger some random error rendering some page, e.g. by inserting a bogus `$foo = bar();` in the code.
See that the error page and stacktraces rendered look the same before and after this patch.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Maniphest Tasks: T16372
Differential Revision: https://we.phorge.it/D26541