Fix loading attribute keys with $akIdentifier = false

If $akIdentifier is false, the query executed by AttributeKey->load()
returns random data, since a text column is checked agains 0
(see for instance http://sqlfiddle.com/#!2/38237/2)
Merge pull request #1598 from mlocati/fix-attributekey-load

Fix loading attribute keys with $akIdentifier = false