Support for sites without a pathPrefix

[umstek]

The boilerplate code doesn't allow an empty pathPrefix in data/siteConfig.js but allows a / which will be regarded as "" (empty) somewhere and, there are also some code which will convert "" (empty) pathPrefix to a /.
These incompatibilities lead to URLs like https://org.github.io//test-url (note the double slash) in SEO and social share links.

[Songkeys]

@Vagr9K
When run build, it throws

ValidationError: child "pathPrefix" fails because ["pathPrefix" is not allowed to be empty]

Could this validator be disabled? Apparently only changing the parser in SEO component is not a good idea. For example, somewhere else (in Disqus), it also uses urljoin to get the url which also leads to https://org.github.io//test-url.

[Vagr9K]

Apparently url-join doesn't de-duplicate slashes (jfromaniello/url-join#42).

In any case, you can either use an empty string or / as a pathPrefix.
GatsbyJS validation will be handled automatically.

Some additional validations to prevent incorrectly generated URLs were added.

Thanks for the report!