CMake: skip server entries without empty build system information

Summary:
In one of my work projects, the CMake 3.13.1 server gave fileGroups
responses without any useful CMakeFile data. This then was stored
in our lookup hash used by CMakeManager::fileInformation.

Then later, when we queried for these files, we found a match,
albeit empty, and returned that leading to completely broken C++
language support.

To prevent this issue, we now skip useless empty CMakeFile entries.
This fixes the specific scenario I'm seeing in my work project.

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D17679