Why is the version number blank for the Joomla! library?

From Joomla! Documentation

There was a bug with the starting data for the Joomla! library row in the #__extensions table. This causes the version, date, and author information to be blank when you navigate to Extension Manager → Manage and look at the Joomla! Web Application Framework.

To correct this, simply select the Joomla! Web Application Framework and click on the "Refresh Cache" button.