GenericProjectManager: Do not crash on removing all files

When removing all files from a generic project QC crashed on
trying to access the first item of the empty file list.

Change-Id: I85045bf126f4e12575305466f0f4a6c4191176d3
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/genericprojectmanager/genericproject.h

@@ -66,7 +66,7 @@ private:
     QStringList processEntries(const QStringList &paths,
                                QHash<QString, QString> *map = nullptr) const;
 
-    static QString findCommonSourceRoot(const QStringList &list);
+    Utils::FilePath findCommonSourceRoot();
     void refreshCppCodeModel();
     void updateDeploymentData();
     void activeTargetWasChanged();