Call the getProp() in __isset because the get method populates the property with its true value.

We want this to be true:
$first = empty($entity->var);
$entity->var;
$first === empty($entity->var);

Which it wouldn't be before this change, if there was a getVar() method.
__get should return NULL in the faulty case of having a  = self::NULL but no getVar() method.
Clarify the the comments a bit more, to reflect the discussion in the pull request
Merge pull request #1 from odensecentralbibliotek/isset

Clarify the the comments a bit more
Merge pull request #1 from ding2tal/isset

Return true even though the variables value is self::NULL.