[Tizen] [Runtime] Fix the issue that widget apps fails to install without version attribute.

It reports installation crash if a widget app doesn't contains
widget 'version' attribute in the config.xml file. The root cause is
Version::GetString() always assumes a Version instance is valid, in
other words, not an empty string.

Due to the 'version' attribute could be optional for widget app, a
solution to fix this issue is we add check in
ApplicationData::VersionString().

BUG=XWALK-1226
Merge pull request #1724 from shaochangbin/fix-bugs

[Tizen] [Runtime] Fix the issue that widget apps fails to install without version attribute.