🐛 fix(header): don't fail if custom_subset absent

[xvello]

Following the install instructions on an empty zola project gives the following error:

Error: Failed to build the site
Error: Failed to render pager 1
Error: Reason: Failed to render 'index.html' (error happened in 'base.html').
Error: Reason: Variable `config.extra.custom_subset` not found in context while rendering 'partials/header.html'

Adding this config option to my config.toml fixes it, but the template should be resilient to that option being absent, like it does with other options.

[welpo]

Great catch. Thank you!