ENYO-2466: Accessibility: When the unit of SimpleIntegerPicker is null, screen reader reads value with 'null' string.
ENYO-2466: Accessibility: When the unit of SimpleIntegerPicker is null, screen reader reads value with 'null' string.

Issue
When the unit set to null, it read 'value' + 'null'

Cause
Label string is set with this.value + ' ' +  this.unit

Fix
Add null checking for this.unit

https://jira2.lgsvl.com/browse/ENYO-2466
Enyo-DCO-1.1-Signed-off-by: Jaewon Jang <jaewon98.jang@lgepartner.com>

enyojs/moonstone