fix crash on navigation crash, when clicking on '1' container of clang's libc++ std::1::string

Summary:
On OSX, with XCode's clang, the code browser would show "std::string" being a part of container "__1" (implementation detail of libc++ I believe)

Clicking this "__1" crashed, repeatably.

This fix simply checks if the value is null, and does not try to access it.

Reviewers: mwolff, KDevelop

Reviewed By: mwolff, KDevelop

Subscribers: kdevelop-devel

Projects: KDevelop

Differential Revision: https://phabricator.kde.org/D564