shell_exec() null cannot be used to detect failures

[dbrekelmans]

The Safe\shell_exec function currently throws an exception when null is returned from \shell_exec. A null return is not always equal to an error occording to the docs.

https://www.php.net/manual/en/function.shell-exec.php

This function can return null both when an error occurs or the program produces no output. It is not possible to detect execution failures using this function.

[shish]

Also \shell_exec returns false for a different category of error, which Safe\shell_exec doesn't handle at all :(

[shish]

Reading this some more, I think what it's saying is "null can mean no-output or subprocess-error, and neither is considered an error by shell_exec - but if it returns false, that actually is an error" -- so the problem is Safe is detecting shell_exec as a "returns null on error" function when it's actually a "returns false on error" function, and that's something we can fix