Update SafeProcessHandle.Unix.cs

[Neo-vortex]

fix: Throw appropriate exception when no shell-open utility is found

fixes : #96565

Problem

Process.GetPathToOpenFile() can return null when no shell-open utility (e.g. xdg-open, open) is found on the system. The return value was previously used without a null check, which could lead to a null reference further down the call path.

Changes

Added a null check on the return value of GetPathToOpenFile() that throws a Win32Exception with ERROR_NO_ASSOCIATION when no utility is found.

filename = Process.GetPathToOpenFile()
    ?? throw new Win32Exception(Interop.Errors.ERROR_NO_ASSOCIATION);

[adamsitnik]

@Neo-vortex thank you for your contribution.

Could you please add a test similar to this one? Then on distros where xdg-open and similar are not available (IIRC one would be Ubuntu run via WSL), we can test the change.

Thanks!