Feature user check should check for null (similar to how user story check does)

See
[link](https://github.com/RallyTechServices/summary-dashboards/blob/master/work-item-field-issues/src/javascript/feature-validation.js#L40)

should be the same as 

[link](https://github.com/RallyTechServices/summary-dashboards/blob/master/work-item-field-issues/src/javascript/story-validation.js#L41)
