Fix crash when ESP is not mounted

[WhyNotHugo]

Generating a new bundle with an unmounted ESP would crash trying to find one, even if one was explicitly specified.

This was due to Bundle instances always being created with a discovered ESP path. However, this path is always overwritten based on the value provided by the user, meaning that this default is unused and always overwritten.

This changes NewBundle to simply use an empty ESP if finding one fails. This has no impact on sbctl itself. This change COULD impact applications using this codes as a library, but this only affects code that previously panicked.

Fixes: https://github.com/Foxboron/sbctl/issues/132