Fix the following error in ArcanistUnitConsoleRenderer.php under PHP 8.1:
```
strlen(): Passing null to parameter #1 ($string) of type string is deprecated
```
Stack trace:
```
EXCEPTION: (RuntimeException) strlen(): Passing null to parameter #1 ($string) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=97e163187418, custom=4)
#0 PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/unit/renderer/ArcanistUnitConsoleRenderer.php:15]
#1 ArcanistUnitConsoleRenderer::renderUnitResult(ArcanistUnitTestResult) called at [<arcanist>/src/workflow/ArcanistUnitWorkflow.php:190]
#2 ArcanistUnitWorkflow::run() called at [<arcanist>/scripts/arcanist.php:427]
```