Passing an empty string to `file_exists()` is deprecated behavior since PHP 8.1.
The already existing `if ($file)` check in `AphrontStackTraceView` implies that `$file` can indeed be empty.
Thus add another such check higher up in that class to avoid deprecation warnings when rendering stacktraces.
```
ERROR 8192: file_exists(): Passing null to parameter #1 ($filename) of type string is deprecated at [/var/www/html/phorge/arcanist/src/filesystem/Filesystem.php:1068];
#0 file_exists(NULL) called at [<arcanist>/src/filesystem/Filesystem.php:1068];
#1 Filesystem::pathExists(NULL) called at [<arcanist>/src/filesystem/Filesystem.php:1180];
#2 Filesystem::assertExists(NULL) called at [<arcanist>/src/filesystem/Filesystem.php:1020];
#3 Filesystem::isDescendant(NULL, string) called at [<phorge>/src/view/widget/AphrontStackTraceView.php:33];
```
Closes T15881