dxf export: either use rgb or palette color (followup 9961095)

Author: jef-n

src/core/dxf/qgsdxfexport.cpp

@@ -431,10 +431,10 @@ void QgsDxfExport::writeGroup( QColor color, int exactMatchCode, int rgbCode, in
     minDist = dist;
   }
 
-  writeGroup( exactMatchCode, minDistAt );
   if ( minDist == 0 && color.alpha() == 255 && minDistAt != 7 )
   {
     // exact full opaque match, not black/white
+    writeGroup( exactMatchCode, minDistAt );
     return;
   }