[Tizen] Fix of backendlib - pkg_plugin_get_app_detail_info_from_package should validate widget path

If pkgmgr-server passes path that is not valid widget path,
this will lead to crash of pkgmgr-server. Function
pkg_plugin_get_app_detail_info_from_package of backendlib
should check if valid package data can be received from
given path.

BUG=XWALK-2683
I assumed it's not allowed according to current usage of nullptr.
Will fix.