Fix Process::INITIAL_PWD for non-existent path

[straight-shoota]

This patch changes runtime behaviour to gracefully handle if the initial working directory does not exist.

• Exception::CallStack just ignores it and shows full paths. The initial working directory is just used to shorten paths, no big deal if that's not possible.
• executable_path_impl on openbsd returns nil if the initial working directory does not exist. It's impossible to find the executable that way.

Resolves #7198

[oprypin]

So should the test be totally disabled for win32? "Pending" means it can be made to work, but seems like this one can't

[straight-shoota]

So far we only know it apparently can't work this way. But it might be doable in a different way...? Not sure about that. And in doubt, I'd rather leave it pending than entirely forget about it.

[HertzDevil]

From https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/rmdir-wrmdir?view=msvc-170#remarks:

The directory must be empty, and it must not be the current working directory or the root directory.

And https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/rmdir:

You can't use the rmdir command to delete the current directory. If you attempt to delete the current directory, the following error message appears:

The process can't access the file because it is being used by another process.

If you receive this error message, you must change to a different directory (not a subdirectory of the current directory), and then try again.

[oprypin]

Hmm isn't this a breaking change - changing the type of INITIAL_PWD to nilable

[straight-shoota]

Yeah, but it's not part of the public API (nodoc). So that should be fine.