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();

eeschema/sch_plotter.cpp

@@ -134,6 +134,14 @@ void SCH_PLOTTER::createPDFFile( const SCH_PLOT_OPTS& aPlotOpts,
         sheetList.push_back( m_schematic->CurrentSheet() );
     }
 
+    if( sheetList.empty() )
+    {
+        if( aReporter )
+            aReporter->Report( _( "No sheets to plot." ), RPT_SEVERITY_ERROR );
+
+        return;
+    }
+
     wxCHECK( m_schematic, /* void */ );
 
     // Allocate the plotter and set the job level parameter