CppTools: Remove QTC_ASSERT in CppModelManager::editorDocument()

This fixes

    SOFT ASSERT: "!filePath.isEmpty()" in file
    /home/nik/dev/creator/creator-ut/src/plugins/cpptools/cppmodelmanager.cpp,
    line 467

which can be triggered by e.g. a "git show" document
(onCurrentEditorChanged does not check for an empty file path).

Change-Id: If4ed8e552069b290cb4ac93da52427b7ed2b91e8
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>