Server permits empty-named users

[jamoore]

=> select * from experimenter where omename = '';
  id  | permissions | email | firstname | institution | lastname | middlename | omename | version | external_id 
------+-------------+-------+-----------+-------------+----------+------------+---------+---------+-------------
 1507 |        -103 |       |           |             |          |            |         |       0 |            
(1 row)

this has particular implications on various pieces of code. DropBoxDirectoryCheck, for exampe, assumes a minimum length of one: source:trunk/components/server/src/ome/services/util/DropBoxDirectoryCheck.java@3765#L85

[jmoore]

See #701