Homestyx hydra

R1:cb9fcf18d5c3

Fix PHP 8.1 "strlen(null)" exception trying to stop maintenance mode of Diffusion repo

Summary:
`strlen()` was used in Phabricator to check if a generic value is a non-empty string.
This behavior is deprecated since PHP 8.1.

Use `isset` instead to check if the argument "start" was passed to the CLI command.

```
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.
Repository: R1 hydra
Commit Date: Jul 18 2023