Multisite sites without a path redirect to signup page

[Ipstenu]

From ​https://wordpress.org/support/topic/after-update-to-39-one-sub-site-directs-to-wp-signupphpnew?replies=4&view=all

Pre 3.9, WordPress would treat an empty 'path' variable for the site as a /

As of 3.9, this is no longer the case, and a blank path will act as if the site does not exist.

It's a simple fix in the admin section, but this is a regression from 3.8

[JPry]

A more comprehensive patch

[JPry]

Alternate method of fixing the reported issue

[danielkanchev]

This is not related to the sub-site issue but if a single WP site not part of a network has its "home" and "siteurl" set to 'NULL' (empty string) then the WP displays "Error establishing a database connection". Probably the fix for the sub-site issue should be a universal fix for this issue as well.