Correctly handle setting the `org-hugo-base-dir` variable

[vedang]

When adding a link to an image, org-hugo-link gets called, which fails because org-hugo-base-dir is not set.

Currently, I have set this as part of the ox-neuron backend to org-neuron-base-dir as a workaround, but I need to handle it such that it should work even if it is not defined.