Load the previous project if an empty string is the argument

Previously it would just not open any project at all. This
was a problem for some launchers, which would pass an empty string
when no file is passed by the user.

Fixes https://gitlab.com/kicad/code/kicad/issues/5055

kicad/kicad.cpp

@@ -139,10 +139,9 @@ bool PGM_KICAD::OnPgmInit()
     wxString projToLoad;
 
     if( App().argc > 1 )
-    {
         projToLoad = App().argv[1];
-    }
-    else if( GetFileHistory().GetCount() )
+
+    if( projToLoad.IsEmpty() && GetFileHistory().GetCount() )
     {
         wxString last_pro = GetFileHistory().GetHistoryFile( 0 );