Stop warnings on using QFileInfo without proper filename

 * Stop warnings about using QFileInfo without a proper filename.

   Turns out that the filename of the project can still be empty
   when calling these methods from the new project wizard (before
   the first page is done).

src/plugins/projectexplorer/project.cpp

@@ -234,6 +234,8 @@ QString Project::projectDirectory() const
 
 QString Project::projectDirectory(const QString &proFile)
 {
+    if (proFile.isEmpty())
+        return QString();
     QFileInfo info(proFile);
     return info.absoluteDir().path();
 }

src/plugins/qt4projectmanager/qt4project.cpp

@@ -793,6 +793,8 @@ QString Qt4Project::defaultTopLevelBuildDirectory() const
 
 QString Qt4Project::defaultTopLevelBuildDirectory(const QString &profilePath)
 {
+    if (profilePath.isEmpty())
+        return QString();
     QFileInfo info(profilePath);
     return QDir(projectDirectory(profilePath) + QLatin1String("/../") + info.baseName() + QLatin1String("-build")).absolutePath();
 }