Prevent attempt at plotting invalid sheets

If the number does not exist in the schematic, don't attempt plotting

Fixes https://gitlab.com/kicad/code/kicad/-/issues/19039

common/plotters/PDF_plotter.cpp

@@ -1110,7 +1110,9 @@ int PDF_PLOTTER::emitOutline()
 
 bool PDF_PLOTTER::EndPlot()
 {
-    wxASSERT( m_outputFile );
+    // We can end up here if there was nothing to plot
+    if( !m_outputFile )
+        return false;
 
     // Close the current page (often the only one)
     ClosePage();