Fix Web UI functional tests failing on getAttribute

Web UI functional tests failing on all branches with a common set of failures (see master, 3.0.x and 2.4.x). It seems to me that the behavior of getAttribute on webdriverio changed. It now seems to return the same result as calling the actual getAttribute method on the node. This seems to be consistent with the documentation, but before, it returned "true" and null for boolean attributes and now it returns an empty string and null respectively.