NavigationTabButton: Fix icon.source detection
The `icon.source` property has `url` type, and can not be compared directly to an empty string. See also: qqc2-desktop-style@e74b8530
The `icon.source` property has `url` type, and can not be compared directly to an empty string. See also: qqc2-desktop-style@e74b8530
mentioned in commit 585de327