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).

Let's fix this.

Bug-tracker: http://www.concrete5.org/developers/bugs/5-6-2-1/attributekey-load-returns-wrong-data-if-akidentifier-is-false/