Author: David Mudrák
MDL-63174 user: user_create_user to throw exception on empty username
The core API function user_create_user() did not check the case when the
given username was empty.
Also adding a missing string 'usernamelowercase' for the existing lower
case check and unit tests.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=f6be98b9423f18451d7e84865c7f54064fafbdec
http://github.com/moodle/moodle/commit/f6be98b9423f18451d7e84865c7f54064fafbdec
+ 3.5 en [invalidusernameblank,core_error]
+ 3.5 en [usernamelowercase,core_error]