Fix QUiLoader not applying empty string properties.

Introduced by 6b02fc50ec6a5236f628b08ebeca50e62b22f362 .

Task-number: QTBUG-33130
Change-Id: Ida5ac8c3bf043cb04650680a2fc3777eff3b3f3f
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>