[Tizen][Runtime Model] Fix installer crash if icon is missing from manifest.

Installation crashes if the icon field is missing from manifest.json.
However, this behavior is not correct.

The installation should not crash even though no icon is provideded.

BUG=https://crosswalk-project.org/jira/browse/XWALK-110
Merge pull request #1062 from shaochangbin/issues

[Tizen][Runtime Model] Fix installer crash if icon is missing from manifest.