Homestyx hydra

R1:59428f64ca2c

Fix PHP 8.1 "file_exists(null)" exception rendering AphrontStackTraceView

Summary:
Passing null 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…
Repository: R1 hydra
Commit Date: Jul 11 2024